I work in a department of around 100 people in a company of over 100,000. Our company-wide instruction is to reboot once a week, that is it.
My record with leaving my own personal PC on, is 2 weeks on Windows XP. A friend of mine once left his laptop, running Windows 98, running for 3 weeks without rebooting and had no problems.
To the point:
If you want your PC to last the longest time possible, you need
1) maximum cooling: use a temperature monitoring utility on your PC, add fans where necessary
2) clean air: install filters on fans, if there are non present already.
3) invest in an uninterruptable power supply (UPS): this will filter out the spikes and troughs in your mains power supply, and ensure there are no "hard" turn-offs when you get brown-outs and black-outs.
4) If you use a PC in a benign environment (constant temperature and humidity, i.e. an airconditioned room) then it will last for a minimum of 15 years in a constant powered-on state.
However, there is no need to do all that. Your new PC is likely to last a good 3 or 4 years with no errors if you turn it on and off when you need it, whilst parked in a typical room.