how long can you stand using your computer?

I am computer network administrator. I use computers, my mail server and other computers at work for 9 and a half hours from 10 AM to 7:30. When I get home from work, after having dinner, watching television and relaxing a bit I again use computer to answer only a few discussions here on MyLot that intrests me, check my e-mail etc. In one day I use computer for about 12 to 13 hours.

I am usually on the computer about 8 hours. I have other things to do otherwise it would probably be longer.