I have to honestly say i purchase approximately 1/2 a case (case= 6 boxes). Im also more of a competitive player which means i have to try to avoid purchasing cards around release as the prices tend to be more inflated, and when a card is in the latest and greatest deck it also see a massive price increase.
I have noticed that in 1/2 a case i get approximately playsets of all the commons and playsets of the uncommons as well (except in some situations). While the rares i tend to avg about 3-4 of each and the mythics i tend to get 1-2 but these are all averages.
I know im an exception to the above three but i can't stand not opening packs for some reason i just find it oddly amusing and fun.