How long must attorneys at most law firms wait before being considered for partner?

In big law firms (i.e. 50+), the current trend is to be considered for non-equity partnership somewhere around 7-9 years, and equity partnership within a few years of that.

However, this is all somewhat dynamic; the partner system was seriously injured by the crash in the legal marketplace in 2008.
Additionally,  The current trend is 7-9 years for partnership, although what that means varies significantly from firm to firm.  Many larger firms give lawyers the title of partner even though they are only "income partners," i.e., salaried lawyers who do not share in the firm's profits, and often work on contracts.  In firms with multi-tiered partnerships, equity partnership may only come, if it comes at all, 4-6 years after initially making partner.  In firms where there is only one tier of partnership, i.e., the title of partner is reserved only for equity holders, it is not unknown for associates to practice 10+ years before making partner.  Some of these firms will give titles like "counsel" or "senior attorney" to associates after 7 years or so until they become partners.

And yes, just about every large firm makes a partner or two "early" every few years--at some larger firms that make a few dozen partners per year, this may be an annual occurrence.  My current firm made a lawyer a partner after 5 years, and at my prior firm, we had some lawyers make it in 5 years, as well.  Some lawyers are just exceptionally skilled early in their careers, and some lawyers are exceptionally marketable early in their careers.  In either case, it wouldn't be smart for a firm to run the risk of losing people that they've already recognized as "keepers" within their system. 

That all being said, You don't know that there is a list of individuals who made partner earlier than average, nor that such a list would be particularly useful.  Making partner at any time is a personal achievement, but it does not mean that work/competition is done.  It could be analogized to completing the first stage of a triathlon; it's great that you made it, but there's still a long way to go.  If anything, most partners work harderthan they did as associates:  for income (salaried) partners, there's the challenge of developing a book of business to try to become an equity partner, and for equity partners, compensation is largely based on the work that you generate, so there's a constant challenge to bring money through the door.  Speaking personally as an equity partner (who is now trying to grow his share), in addition to my normal business work, I also now spend significant amounts of time on business development and attention to the marketing/economics of my firm, none of which earns me a dime, at least not initially.  The bottom line is that if you look at random large firms--who is running them, who is a rainmaker, who is a nationally recognized name, etc.--most of those people made partner 6-9 years after law school, and kept on working.  Making partner early means that you've climbed another rung on the ladder of success (and before your classmates), but it does not mean that you're on easy street for life.