Some advice with 'blind' fermentation of a cherry melomel

Go easy, this is my first proper batch.

I've previously brewed beers in either large 5 gallon batches, or as testers in small glass demijohns. As a go-between I've bought a few new buckets that are about 3 gallon to try fermenting honey in.

My problem is that they're ever so slightly leaky to the air. That is, my airlock's showing no activity, but I can literally see (and hear..) the bubbles inside the liquid firing away.

I'm not too worried as I think there's a ton of CO2 built up in there, but I'd like to know how best to gauge when fermentation might be over - any experience?

I've never had a problem with things going off before, I don't want this to be my first! I could just leave it sat there for a month; my only worry is that the cherries in there as a fruit might start acting shady - I don't want to have off flavours if I can help it.

Any guidance, or sharing of experience much appreciated.