It can last a long time, but if there's
water in the
core it'll get spongier and spongier. My
Irwin 27 didn't even have
core there--just a piece of
plywood glued to the underside, so it was dead easy to pop it off and properly glue in another piece and glass it over.
If the sole where the tiller comes though is part of the support system for the
rudder shaft, then it would be wise to fix that before the
rudder goes wonky.
When you drop the rudder to do that is also a good time to inspect the shaft where it goes into the
fiberglass rudder body: this is an area where shafts commonly corrode and break.