IMO - the good years of te SG Franchise ended after Season 6 of Star Gate SG-1. (It was GREAT when it was on Showtime; and it's first season after Sci-Fi picked it up was okay, but it went downhill fast when the cast switched to Ben Brodwer and the rest of the former FarScape cast.
I would take it just a little bit past that, to the end of Season Eight, specifically the episode 'Threads'. Had they decided to end the show with that ep, I could've lived with it, as it had a nice feeling of closure to it, yet at the same time, you knew that their adventures would continue. But it brought to an end the eight years of their conflict with the G'oauld, and the last scene was of SG-1 at O'Neill's place.
It could've ended there nicely. To be honest, I would've preferred it as a resolution to the finale that we got in Season 10.