The lifespan of engine
mounts can vary widely, depending on the type of mounts. The manufacturer probably has a recommendation for when to check and replace.
If your engine's jumping around, or there's pronounced vibrations, obviously the mounts are one thing to check. If you've got the prop shaft disconnected for some other service
(eg change cutless bearing), that's a good opportunity to rock your engine to look for gaps or breaks in the mount. You can also lever the engine a bit (carefully) to check.
I 'apprenticed' a bit with a boat mechanic
one spring, and I was surprised at the apparently short life and fragility of some mounts (-cough-Yanmar-cough-), especially considering the cost and effort to replace. We don't see car or truck engine-mounts failing as frequently. Is anyone aware of an aftermarket marine
engine mount that performs AND lasts?