I don't recall coming across anything in the base book that dealt with how fast credits should be accquired. I realize that this system is more flexible and is encouraging different groups to play their own style however some GM's may be at a loss without some rough guidelines so I would suggest including one line in the book maybe bundled in with accquiring xp about a normal, slow, and fast rate of accquiring credits.
In our games it has ussually been around 2500 a player (from various sources, bounties, selling items, rewards) with occasionally a little more if they go out of their way to earn it. That may seem like a lot but when you have ship repairs, and fuel to pay for along with saving for new weapons and gear I think it has been pretty reasonable.