How often should I dust out my power supply?

Often actually using the warranty is such a hassle that it is far preferable to do what you can to take care of it yourself. You should not open it if you are not familiar with working around high voltages and by familiar, I mean you have a sincere respect for the danger, not simply feel confident that nothing bad will happen to you. But if you never open one up, then how will you learn?
You can make an adaptor to attach a straw to a regular vacuum cleaner and remove some of the dust through the grill. (Look up how to do this because if you do it wrong, your vacuum cleaner might overheat, but most have a safety trip which will reset after an hour.) You should definitely unscrew it from your computer's case if you need to. The power supply is inside its own case that is simply screwed to the computer case. It's not really yours until you void the warranty. That's why if you send it in on the warranty, they have the option to replace it with a refurbished model. If it breaks, you can learn to fix it, if you want to. Otherwise, get what you can with a straw and don't worry about it.

In general, the fact you can feel hot air blowing out is a good sign. When too much dust covers the parts then the heat won't transfer and then instead of blowing out hot air, it will boil an electrolytic capacitor or several.
It's good for your computer to get warm because that keeps out humidity. I am in a humid environment, so I try to keep mine running all the time. Turning the computer on and off actually causes more wear than leaving it on, in theory. The temperature fluctuation causes expansion and contraction of the joints. Keeping your computer at a constant temperature, even if it feels hot to you, is better than letting the temperature fluctuate.
Of course you can't help that it is going to fuctuate as you use it. The best you can do is reduce the range of fluctuation.
Trying to keep it too cool can increase the range of fluctuation. You want the computer to be comfortable, and just as you don't set your thermostat to freezing just because it's hot outside, so too, all you want to do is provide a way for your computer to maintain a reasonable temperature. It is what, a 300Watt power supply? Compare that to a 300Watt incandescent bulb or a 300Watt heater. You can expect it to put out some heat. On the inside, it is feeling fine. If it feels cool coming out, then I would worry something is wrong.

Bottom line: 3 months or as needed is a good maintenance schedule.