My home PC is an AMD and I've had it for about a year, in this time it has been on in all honesty about 11 of those 12 months constantly. I've had no problems at all, although now the machine is getting a little dated.
My next machine will be an AMD thats for sure. Im not sure how many hours that 11 months turns into but the machine is still going strong!
We have a web server in our office with an AMD processor. It's been running constantly for about 10 months now (with the occasional re-boot for patches and fixes).
It really needs upgraded cos we often get 100% CPU utilization when hitting it hard, but it's never given us any problems at all. It's the first AMD processor we've ever used and we wouldn't shy away from using them again.
It runs in an Antec 830 case (thoroughly recommend it - we're now using it for ALL of our PC's and a couple of servers in the office) with 4 fans. One old server case had 9 fans installed and we had constant thermal problems until we swapped the hardware to an Antec.
Sometimes the CPU reaches 30C and the case internal temp sometimes reaches 26C. Don't know how accurate the thermal monitors are. Usually they sit at around 25C and 22C respectively.
I wouldn't worry too much about using an AMD constantly. We've never experienced any problems.
You do realize that "Sods Law" will now take effect
My XP 1800+ runs at about 40 Celsius or slightly below at a room temperature of about 24 Celsius. I know I haven't got my fans set up right (I've gotta turn one or two round so they're exhaust and not suction), meaning the airflow in the case isn't optimal, but I have a HUGE servercase (room for 8 harddrives) with a load of fans and a digidoc system built in and it's highly recommended (I also quite like the neon lighting and the window in my case ).
My Dual CPU's AMD run at about 35-38 degrees.. air conditioned data center with 5 Fans in the (large server case), there are about 8 other dual amd's right beside mine and all have been running without missing a beat since last November.. Intel do run cooler but I haven't seen or experienced any failures from AMD as yet due to heat .. Sods law will strike you bet
You can run a AMD based system as long as you want. I leave my online for days in a row, just for the heck of it(well im actually lazy, and hate booting up). The only problem with the AMD processors is, they dont have any intergrated heat protection. However a good deal of the motherboards out DO have heat protection. With heat protection, I mean lets say the heat sink becomes unlatched. The only way a Athlon would fry is if, the heat sink fan falls off, and there is not thermal protection intergrated into the motherboard. THIS IS RARE