The lifetime of electrolytic caps depends on their temperature rating and their actual operating temperature.
Operating them at their rated temp will fail most types after 2000 hours, any 10°C lower doubles that. E.g. a 85° type run @40°
should last appr. 2000 x 2^4.5 ~=45,000 hours (~5 years 24/7). If you mismatch polarization (like on the A3640 or the audio circuits), the cap will die much sooner, depending on current. Of course, running a badly ventilated and hot system will have a bad impact on caps life, as well as loading them with high currents (you need low ESR types for that). Some caps used in PCs and other equipment are filled with bad electrolyte and
fail after only a few months or years.
You could just use that rule for not-in-use/stored caps with, say 25°C, but I think they age a lot slower when not in use.
Anyway, it's a good idea to only use 105° caps for anything valuable.
