tube life on preamp tubes is extremely long. Most people only swap them out if they are microphonic or if they want to tweak their tone. Power tubes last anywhere between 6 months and two years, depending on use. I get about a year out of my power tubes, and that's with two practices a week and 2-5 gigs a month. I could probably be longer but I try and service my amp reasonably regularly. How they last for you depends on your sttle and workload.
If you get one, plan to put away some cash in future for a qualified technician to look it over somewhere down the line. Always a good idea. Find out who the good amp tech guy is in your town and make friends. A well made, decent brand amp and a good relationship with your tech are worth their weight in gold.
As for overheating, I've never had it happen. Ask some other guys about their war stories.