So I'm curious as to when people feel Once should have ended, because I have seen people have all 3 of these opinions expressed by people. Here are the main points I've seen for them:
Season 6 - This was written to be a finale of Emma's story, and would allow for the show to end with all the main cast still as main cast.
Season 7 - This allowed for us to see things like Henry as an adult with his own family, Rumple finally letting go of the Dark One, Regina and Zelena getting some sort of romantic partner, etc. Basically it wraps up things that Season 6 didn't have time to do. It also allowed for a few stories that were missing in Season 1 - 6 to be done, like The Frog Prince/The Princess and The Frog.
Season 8 - Assuming that ABC would tell Adam and Eddy this would be the final season a head of time, this would give them a whole season to be planned as an ending. It would also mean that Season 7 wasn't a weird one off season.
I was personally hoping for the Season 8 as a final season, because it would have given us one more year of the show, and given Adam and Eddy a whole season to work toward an ending. But let me know what you think!