What value for a Rfc2898 (PKBDF2) is overkill?
what value or iteration count is enough?


So in NIST SP 800-63B a baseline of 10,000 iterations is recommended. Going below that is a really bad idea.

Now of course the actual guidance is "do as many iterations as you can afford", which usually amounts to "do as many as your users are willing to wait for". If 65,000 already take 1 second and your users will complain enough about that to go for a different application lower the value. If they're patiently waiting 5 seconds, go for the iteration count that takes 5 seconds.