Life of amp internals

In a poweramp what is the range of expected life of the parts - resistors, transformers. For example 10-20 years.
I think that capacitors are the first things to go. Somewhere in the 10-20 year range depending on the particular cap. They can be easily replaced though. Other than that, deterioration will probably be caused by environmental conditions rather than due to usage over time. The time it takes for environmental deterioration such as oxidation to take place will depend on the conditions in which it is kept. Many audiophiles change their equipment so often, they probably don't have it long enough for significant deterioration.

When I think about how many audiophiles have their system down in their basement, I wonder if they've ever popped the lid on a component and looked inside after a couple of years. I have some filing cabinets in the basement. Even with dehumidifiers going, I'll pull something out of the cabinet and the staples have turned to rust after a year or two. It's frightening to think what's happpening to the innards of components.
The heat equipment generates should help dry it out in the basement much like a damp chaser. I have some ss gear that has been operating 30 years. I would be more worried about power surges and swells taking the thing out.
Mark is right. Caps are the first to go. Although my McIntosh MC240 had caps that were 40 years old and it still worked but a complete overhaul made a huge difference. Everything else will should be fine much longer.

Electrolytic caps have a half life of about 20 years- in that time about half of them will have failed. The transistor amp above that is 30 years old has a seriously degraded power supply; despite that the amp still plays although at reduced performance.

Transistors that are mounted on heatsinks face the danger of the heatsink compounds perishing, leaving the transistor unable to transfer all their heat to the heatsink. Its a Good Idea to remount the transistors with fresh materials about once every 15 years or so.

Corrosion and how well parts are sealed will determine how long the other parts in an amp will last but once you've gotten to 30 years or so all bets are off.