You won't find a single number for how long they last but 10-15 years is a good point to start checking them for shorting, or just replace them. It's probably the cheapest insurance to keep vintage/expensive gear working without launching tubes or transformers. If it sits idle for a long time or is run really hard, they go faster than under normal use. Fortunately, decent electrolytic caps aren't terribly expensive but the best coupling/audio caps are pretty pricey.
Get the amp you want- if it's ready for a re-cap, offer less. At that point, re-cap it and enjoy it.