My PC's been on for about 18 months now. I never turn it off. Hardware failure is most likely to occur when you start a PC up. In businesses it is commonplace to leave equipment running to minimise hardware failure at startup time.
When i had a desktop it used to be ON for alot of days, but now i got a laptop and close it after 12-16hrs.
I used to run up 4-5 days continuously in holidays without any problem...... Now a days it remains on for more than 13-14 hours daily ..
Usually 2-3 weeks ( don't like to switch it off ). May sound crazy, but it does not make any problems for me
I used to turn off my PC at the end of the day. Then one morning a few months ago, while I was powering up, there was a rattling noise that lasted for several minutes. Was this a warning of imminent component failure? Since that time I've been afraid to shut down my computer for more than a few minutes, such as for an occasional reboot or automatic installation of a software update that automatically shuts down the computer when the installation is completed. I then turn the computer back on immediately. (I do, however, turn off the monitor when I'm done for the day.)