I run mine all the time, hardly turn it off, like to leave it playin music at night, helps me sleep , Very seldom turn it off. My old one i had for like 4-5 years, never turned it off, ran strong, til lightening got it .
i leave mine on 24-7-365 but i have in my bios to shut down the harddrives after 15min of idle time to save them on run time also same for my burners.. i have to manualy shut down my monitor though and it is what eats up the most power any way !
cost for running it 24-7 is minimal .. cooling also isnt a problem due to the fact im not gaming or anything so my chips arent doing much either just idle!