The PermGenSize depends to your application and some other parameters.
There is no general answer, you have to analyse the behaviour.
Helpful starting point is the Java GC site http://www.oracle.com/technetwork/java/javase/gc-tuning-6-140523.html
From Peter Johnson is an article here http://www.cmg.org/cgi-bin/search.cgi?q=java+peter+johnson&x=30&y=10
If the problem is reproducable the java tools jconsole, visualgc, jstat or jhat are helpful. We use jProfiler at the moment, but there are also different Java profiling tools available.