It's a bit hard to give a full number to that, more factors then just the processor come into play, but the number is likely high enough that we would never see it in a phone. No phone in the next 5 years (or ever really) should need 6 or 8GB of ram. At some point it's diminishing returns, so unless someone wants to pay more for a phone with extra ram tossed in to make it look good, I would say that 3GB will still be present in next years flagships.
Android likes to chew up ram, it's designed to do that for the most part, but even then I think the speed of RAM is far more important at this point then how much. Samsung was wise to bump of the type of ram in the S6 to LPDDR4 and that will certainly be standard for any flagship going forward.
Phones like the Note 5 may bump to 4GB simply due to the fact that Samsung piles so much stuff onto TouchWiz for the Note line to get it the functionality that people buy it for. Toss another GB of RAM in the S6 would make little difference, I'd think. More efficient processors, faster RAM and storage and strong software optimization (Google & OEM) are going to make a much bigger difference going forward. Android is reaching the point where you shouldn't need to brute force it with deca-core processors and 4 or 5GB or RAM. It just isn't going to make use of it or need it.