I once ran a 486DX 66MHz for 32days, 16hrs, 38mins, 41sec, running Win95. That was the record - it was still up essentially 24x7, running as a file server. My 250W PSU that went with my P3-500MHz is still running 24x7 (with a shiny 'new' 933MHz CPU). I'd say you're alright.
-bZj