50 TURNS!! That is ridiculous. Why would they put something like that in? I thought I saw something by frogboy that said 8-10 turns.
It's NOT 50 turns.
I don't remember when my initial trade deals were but I was just able to talk to my trading partners on turn 46. Might be 10, though. Gonna have to pretty much check each and every turn and see what the exact number is.
Don't know why RevTKS thought it was 50 turns until you could talk to them again, though.
EDIT::: Checking previous Autosaves versus a primary save, it is at least 12 turns.
I was locked out of trading at turn 27 and turn 39 but was able to trade again at turn 43. So at least 12 turns. I'll edit in the exact number later if I peg it down before anyone else answers in the thread. 
SECOND EDIT:: Actually, I can figure this out. As of turn 39. I had 35 turns left on some deals I made, so that means I probably made them on either turn 24 or turn 25. It looks like I can enter new agreements at the start of turn 44 (don't know why I thought I could on turn 43).
So, 20 turns sounds like what the limit is. Still pretty high, and IMO, excessive. But at least it's not nowhere as bad as 50. 