Yeah, I agree with the shared idea of seven seasons. If they planned on 7, I expect at least seven seasons, ignoring ratings and stuff. It might be 8 because of random Frozen insert (Note: I enjoyed it, but everyone knows that it wasn't planned years ago).
In terms of character growth, I think Once should end post season 5 or 6. One of the hilarious things about Once is that it does cyclical character growth very well, but upon close inspection, it's a tad annoying.
Emma can only keep on accepting so much and learning more magic to an extent. Regina goes through the "Will she go evil? Will she find happiness?" issue every season and the others have similar arcs. If Emma has to learn to accept herself, magic, role, or family one more time I'll die of laughing fits.
With Regina "redeemed" and "happy", Emma finding home with her family, and Rumple possibly getting redeemed, I don't think it could go on longer than Rumple's redemption because Regina will get boring, Emma and Snow would be bland, and Rumple would have finally learned his lesson for the fiftieth time.
In terms of Hero's journey and narrative/plot hype, I think the Darkness would be an end type season. However, they can stretch out the Darkness thing for a season or two if they really wanted too. However, they are prone to their time jumps and things that happen off-screen, that it'll probably be cut out.