Tommy's right, It's turning them on/off that causes problems.
How long can you leave a pc on?, well I've got a 486sx 20Mhz, (The first 486 I built), on site, it only does basic Database work and has been on since 1991.
It's not linked to the network , who's 486DX2 50Mhz file server, running on novel, has been running none stop, since, 1992.
Intel Core2Duo 6400 @ 3.4Ghz
4GB Geil 6400 Ultra
3 x 500 Gb Samsung HD
EVGA 8800 Ultra OC