How Long I Can Turn On My Desktop??

Hi Tony 1
as long as your case has adequate cooling there is no reason you can't leave your desktop on 24/7
also that is a poor quality psu with low grade capacitors
for a good psu i would suggest this
http://www.newegg.ca/Product/Product.aspx?Item=N82E16817151074&cm_re=seasonic-_-17-151-074-_-Product

with the current psu you have there are no guarantees , i cant say it will last x amount of time with x amount of use for x amount of years
if you get a psu like i suggest i would say
as long as you like , 24/7 , 3 hours , 12hours , for years it will be fine

For that system you don't need more than a good 300w. Having said that you don't need a new one even with a 450w junker (IMHO junker...but fine for a pc not even pushing 1/2 that).
https://webcache.googleusercontent.com/search?q=cache:APqEstXSY-MJ:www.anandtech.com/show/5541/amd-radeon-hd-7750-radeon-hd-7770-ghz-edition-review/23+&cd=1&hl=en&ct=clnk&gl=us&client=firefox-a
216w at load with and AMD chip.

Remember you have a sub 75w card there and I'm just guessing here, but I'm pretty sure Intel's 22nm at your level is lower than 32nm AMD in the above link. Your PSU probably isn't pushing 200w at load even if you have a lot of other stuff in the box.
http://www.tomshardware.com/charts/cpu-charts-2013/-38-System-Peak-Power,3178.html
Lowest chip in the charts is above yours (3220 @3.3ghz 75w).

http://www.bit-tech.net/hardware/2012/11/26/intel-core-i3-3220-review/2
again above your cpu. Note the power page with i5-3570K (way above you) +GTX 690 (blows your gpu watts away):
http://www.bit-tech.net/hardware/2012/11/26/intel-core-i3-3220-review/7
OC'ed to 5ghz still 267w (stock is 161w). You're below this. Unless you're having a problem you should be fine until you stick in something that draws quite a bit more power. GPU loaded should show another ~100w but whatever...Moving on you'll see what I mean about the 100w.

http://www.bit-tech.net/hardware/2012/06/13/nvidia-geforce-gtx-690-4gb-review/8
690gtx review, just happens to have your card at the top.
Test System

Intel Core i5-2500K (operating at 3.3GHz – 33 x 100MHz)
Asus P8P67 motherboard (Intel P67)
2 x 2GB Corsair 1,600MHz DDR3 memory

So i5-2500K+7750+4GB is 121w loaded...Again I don't see how you'd be over 200w with a die shrink to IVY and 1/2 the cores of i5-2500k (and not running as fast). No new PSU needed for now (look at dell 300-350w basic pc's with this type of card as options). AT $33 USD I'd stay on the current PSU until you can afford something like the other poster suggested. Sure I'd like a better PSU, but you don't NEED one unless it's failing. Even the 7950 is only coming in barely over 200w with the same core.

24/7/365 is fine, just like mine assuming proper cooling this is no issue at all It is rarely asleep...LOL. But I do believe in Seasonic, and have a PC Power and Cooling who is made by then anyway. I'm pretty sure that is still the case since they got bought out by ocz.

Drop in the card and burn it in to see if you crap out. I don't believe in buying a psu that's less than $50 This part is so overlooked (but you have plenty of power junk or not). So save and stick with what you have until then. I'm not arguing with the OP choice, just that you don't need one now. 12v lead is weak but I'd still run until you can buy above $50 unless you crap out with the burn in.
http://www.newegg.com/Product/Product.aspx?Item=N82E16814150663&cm_re=radeon_7750-_-14-150-663-_-Product
$87 after rebate if you can buy from newegg. Wide though

http://www.tomshardware.com/reviews/radeon-r7-250x-graphics-card-review,3747-3.html
Would rather have a 250x that blows away 7750 (it's a 7770 really) as you can see.
http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=100007709%208000%204027%20600473873%20600487621%20600473874&IsNodeId=1&ActiveSearchResult=True
or pick anything in there, only R7's 250, 250x, 260x etc. All about the same price as 7750 which I wouldn't buy (why vs. 250x at ~same pricing?).

http://www.newegg.com/Product/Product.aspx?Item=N82E16814131559&ignorebbr=1
$70 after rebate ($90 before)+free game. 640sp (avoid the 384 core models, they suck). 1000/1125mhz boost
http://www.powercolor.com/Global/products_features.asp?id=537#Specification
400w min and I think they're banking on a more powerful cpu here.

Good luck

you are welcome

Yes, you're very welcome. I hope you can pull enough together to get a PSU as mentioned. Seasonic is a nice brand and PC P&C picked them for years for a reason and have proven they have great QC and can meet stringent requirements they asked them for (been using PCP&C for 20yrs). I used to joke you could kill someone with one of these things as they are so heavy (my old 500w was ~6lbs!). It's comic but PSU's can actually be pretty accurately judged by their weight. A vendor used to do that ages ago, but I can't remember the name. They would score in quality (voltage outputs on the rails, regulation etc) at sites like anandtech and here just like they weighed...ROFL. OH, just hit me, DIRECTRON. They did it.
http://www.directron.com/table1.html
It doesn't look like they've kept it up. I take that back, I checked a few of their new psu's and they still list the weight but just don't seem to make an updated list for comparison sake. But as an example:
http://www.directron.com/ra650.html
FSP 650 3.85lbs $73
http://www.directron.com/ea650platinum.html
Antec 650 4.4lbs. $105 -
Check out the old list above showing 500-550 watters running up to 5.45lbs! They did that with fewer cables/connectors back then too. My silencer 750 is easily over 6lbs (ship weight is 7.7lbs). Last I checked they make junk under 350w though, the real deals are above that (everyone needs a cheapo line to sell - still ok, but not BEEFY in any way). The seasonic 400w in that old list is barely under the FSP 650w I listed and the PC Power & Cooling 410w in that old list is a whopping 4.7lbs! They also aren't what they advertise but rather far above it. Like the 500w I had could push 600w peak fairly easily (they even put that on the label). My 750w is an 850w max I think.

More info than you needed to know, but this thread just brought back some memories of how I used to determine the junk just by picking up the PSU boxes at ASI/Supercom etc back then. Antec, Enermax, Seasonic always had some heft to them on mid-higher models. They all skimp on low-end stuff aimed at IGP's and no discrete (meaning 300w and down). PCP&C was a direct vendor for me back then (I was a reseller of theirs). ONE RMA in 8yrs of their PSU's! And it wasn't dead, just a little low on leads and it was for an OCing PC customer (rock solid just low), they adjusted and sent it back 2day air with an apology note saying I might want to pitch their units with a dial on/in them so my OCers could adjust it any way they wanted (they had just announced these models back then) NICE SERVICE. They knew I sold OC machines a lot (step-thermodynamics before they died etc etc) so it made sense for them to comment on it.

Glad you could make sense of what I said as I was hoping to not confuse anyone. If you have an issue just dial down the core speeds in the AMD catalyst drivers and you should survive and still blow away your i3 until you get the psu (if that is what you're doing, turning off IGP and going discrete). Enjoy your new card