You can start getting considered around 10 years, but realistically it's from around 12-15 years. Additionally, some firms trying to be funny now and adding one more layer of director between P and EP, but that doesn't really affect you if you're on the normal track to EP (i.e. 12 years to max 15 years track). The inbetween track is the f you track, where they value they money you bring in but don't view you as important enough to make EP (akin to something like Counsel in int firms).

But take note, alot of the big4 have an up or out culture, meaning if you miss their internal "deadlines" to become Partner you're quite screwed (i.e SA promotion is near lockstep, but if you aren't promoted to Partner from 5-8 PQE you should consider leaving), it's unlikely you will be made EP if you can't even hit the usual 6-7 PQE partner mark.