I would agree with ScrewySqrl, the PSU is the thing you should never go cheap on. a bad PSU could cost you your whole computer if it goes bad.
If you're asking how long you could keep it, it really depends on you. It's not like anyone's gonna take it from you.. haha j/k But seriously it really depends when you feel it would be too obsolete for your purposes and would require an upgrade/new PC to fulfill your needs. On average, computers last people anywhere from 4 to 5 years but they can be used for as long as 10 or so years.