Cels are usually released after production 
ceases, or so I have heard. I don't think there 
is any waiting period after that, though it might 
be a few months before you begin to see them 
regularly here in the states.

The best example of this I can think of currently 
is Pokemon. They are still producing shows in 
Japan, so the artwork has not "officially" been 
released yet - this makes it harder to come by 
and more expensive.