“OFF-in.” NE Ohio, born and raised here.
Off’n.
British, speaking generally with a BBC (i.e. neutralish, to us) accent.
OfTen sounds like someone with a regional accent trying, and failing, to speak ‘proper’.
I’m from California and I say often, but now I saw “offin” has the higher vote. This puzzles me because I thought the only people that said “offin” were from Midwest/Southern states. Like people say “I’m “fixin” to go.”
Meh. Here in WV, people are “fixin’ to” do things all the time! 
Offin, though I rarely use the word. I refer “oftimes.”
WTH? You guys peasants or something?!?
It’s off-tin!
Off’n, closer to the first poll option I guess. There’s certainly no “t” tongue/teeth click, although there is tongue movement upwards, and touching the back of the teeth, at the crucial point.
Scotland.