What is the lifespan of a good SS Amp

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.

I have pair of Meridian active speakers that have astounded me for almost 30 years and the amps have never missed a beat let alone been serviced.
Maybe new caps would be an idea but then they might blow away my good system.
Si

There are too many variables. If properly ventilated, & not continually operated @ rated power, 20 to 30 yrs. isn`t beyond the pale.

Electrolytic caps (the big power supply caps are the primary ones) tend to dry out after 12 - 25 years. Higher operating temps usually result in quicker decline but there are differences among capacitor manufacturers too so it is difficult to generalize. Occasionally you'll see early failure or caps that last beyond 25 years but the above range covers "normal" situations. The electrolytic caps may loose storage capacity before total failure. Replacing these caps will usually cost $100 - $400. The rest of the components in SS amps are likely good for at least 50 years. The caveat is that some transistors are no longer available so if you have catastrophic failure you might be in trouble.