If modern electrolytic caps are not stressed or operated at or over their rated voltage and the temperature is not high, they should last 50 maybe even 100 years. Shelf life is certainly this long.
Most caps in low-power electronics last virtually forever, however their performance will deteriorate over hundreds of years.
Electrolytics used in power amps are another story. Their temperature is usually elevated and they are constantly stressed by AC and high discharge currents, as well as power on current surges. Soft-start helps, so inrush limiters can prolong life. Failure is often catastrophic with the cap shorting and smoking, creating a lot of heat which usually destroys the circuit board. I would say 20-30 years if the amp is well-designed.