What is the lifespan of a good SS Amp

With proper maintenance a solid state amp's life is essentially unlimited too. While it's true transistors wear out and capacitors can go bad, these things can be replaced in a solid state amp just as parts may be replaced in a tube amp.

I had some custom Quad current dumping (transistor) amps that were modified by Steve McCormack (McCormack audio) and they were wonderful for driving subs. After years of use I ordered all new Motorola output devices and replaced them myself.

The results were instant and very gratifying. Like new performance again and actually not all that much expenses to put it back right.

A well made solid state amp should last at least 15 to 20 years. A 4 or 5 year old solid state amp should last you at least another 10 years. I recently had two capacitors replaced on my Acoustic Research Amp, which I bought new in 1969(!!). Actually, the amp developed audible problems late last year, but I didn't get around to taking it in until last month. (Perhaps it lasted such a long time because I have been playing more my newer equipment for the past few decades.)

They last forever basically.

My two McIntosh 2105 Amps are still going strong after 32 years of ownership, and they've never needed service. Never even had a lamp go out, but I did decide to replace them all about 5 years ago. Mark

Depending on the particular design either transistor or tube amps usually run for at least a few years, and can be repaired indefinitely, so long as parts are available.
Tube amps tend to deteriorate so gradually that the user often doesn't realize it. Transistor amps tend to quit abruptly. Tube amp reliability is degraded by the high voltages that the circuits require and by the heat which tubes generate.