Jeans do stretch out, but how much depends on the fabric (is it 100% cotton - rigid, or is it a stretch blend?). You can also shrink jeans and they'll never be as big as they were when you first got them.
It also depends on the actual brand of the jeans, but this can get so detailed very quickly!
Depending on how I want a pair of jeans to wear (tight, or looser and more comfy?) sometimes I will go down a size so that when they stretch out they are still tight. However, this can be quite uncomfortable in the beginning. You can 'wet-stretch' - get the areas you want to stretch out wet, and wear it. It's not too much fun but gets the jeans stretched out a lot faster.
i just posted this at another thread earlier, but i wore my A-pocket for the first time yesterday, it was snug in the morning, but within a couple of hours or so, it was lose! i will need to wash it and dry (for about 20 mins to lose the wrinkles) and hang dry after, then it'll be snug again....