What is the lifespan of a good SS Amp

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.

I have a Harman Kardon int amp purchased in early seventies that's still operational. Pots and switches a little scratchy but if it's in the garage who cares? Had to replace an output transistor, actually one of those output packs they use on less expensive amps.