How many advancements (overall)are your players taking before ranking a character up? Is there an advantage to waiting longer?

You mean switching to a new career? (Rank is a matter of # of experience points gained).

So far, completing a career is seen as a desirable thing. Retaining the career ability and the free skill specializations is compelling.

I agree with Killershrike. If a player is not going the full 10 advances, they are doing something wrong.