I've only done two coach international trips the past year (rest were paid C with no op-upgrade to F) and I managed to get the return for both from Y->C.

First trip (W class) was on a Monday IAD-LHR (no op-up) and returned on a Friday (op-up).

Second trip (standard award ticket) was on a Monday IAD-CDG (no op_up) and returned on a Friday (op-up). On the second trip, once at IAD I had a IAD-PHX flight on a A320 and had 6B. Right before the door closed the gate agent came up to me and said there was one first class seat left and I could have it. That was excellent and was a first for me domestically.

From speaking to some coworkers, Monday-Wednesday are the worst chances for getting them and Friday seems to be the best.