Is there an average number of booster boxes to complete a set?

Hey all, I was wondering if there's any sort of "rule of thumb" or general practice seen in the community on how many booster boxes of a set you need to open to get at least 1 of each card in the set? Ive opened two before and was lucky enough to get them all with just that but that may have been blind luck and that was also before the days of of Mythic rarity (Fifth Dawn).

I recently started getting back into magic since peacing out around the time of the end of the Kamigawa block. Started going to a few Friday Night Magic draft events, enjoyed it, decided to take the plunge on the current standard format but im a sucker for the old stuff that started the "modern" format. Anyway a buddy of mine is trying to get rid of a bunch of booster boxes, having stuff all the way back from 7th edition to the Battle for Zen. He made me an offer on 20 booster boxes from 10 of the newer sets (two of each) at a decent price, but Im not sure really if that would be enough to have a chance at sets full rare count or mythic? Im well aware that two will have me drowning in playsets of commons/uncommons though. Any thoughts/advice?