Based on what I saw, I'd estimate Bitmain was replaceing S5's with S7s for 2 months at most, and possibly with S5+ for a month or two before that (and then likely sold those S5+ as "new" units once they got the S7 into production).
I don't believe for a microsecond that the S7 has been in production for 6+ months.
When you say what you saw what are you referring to? Did you get to see a plant? Just hashing online? What specifically?