With care and a clean environment, a well built amplifier should be expected to give good service for at least 25 years. I have had many vintage amps from the 60's and 70's (both high (McIntosh, Marantz) and mid-fi (Dynaco, Hafler, Pioneer, Sansui, etc) that play as well today as the day they were made.
It's the components with moving parts (Speakers, Tuners, Tape decks, CD Players) that can wear out that will fail much earlier - all are repairable, of course, but not likely to operate trouble free for as long as a preamp or amp will.
As far as I can tell, heat, moisture, dirt, corrosion, and power spikes are the worst enemies, and if they are avoided or managed, your equipment will thrive.