There are a number of threads around here dealing with this issue. The upgrade processor has been broken for ages, the Airbus fleet compounds the problem with only 8 F seats and about 3 other factors come into play that I can think off of the top of my head.

But yeah, you can use T-2 as a pretty good benchmark, 75Ks can usually expect it just a little earlier. That's a good thing, as at T-2 those seats turn into U class and people start buying them at the airport and online out from under those who haven't been upgraded yet.