Hi,
If you keep the switches, pots (potentiometers) and other controls clean and prevent oxidation over time, solid-state electronics are an incredible bargain. I'd estimate at least 15 to 20 years. I talk to customers who have 20-year-old stereo receivers and integrated amps that still work fine, and I've got older solid-state amps that work perfectly but I do clean the switches and pots every five years or so or whenever they get noisy or intermittent.
In theory, a "digital" amp might sound audibly cleaner, but I'd want to do blind instantaneous comparisons with tone-control circuits switched out before I'd make a judgment.
Eventually heat gets to everything--the values of resistors and capacitors may drift over many years--but it's hard to estimate long-term reliability of ICs. Sometimes stray voltage or static electricity can knock those out. But by comparison to tube gear, which begins to deteriorate from the moment you turn it on (and the tubes themselves change with use), solid-state designs are very long-lived.
Regards,
