I keep my seedlings in a smaller container for as long as I can :). I have too many ang they grow too fast once in bigger containers. So I would say, when it starts becoming top heavy for its seedling pot and when the roots are established.
Then I transfer to a quart pot but I only half fill with dirt so it doesn't get too soggy for a long time. If I see roots starting to come out of the pot, then I only pull them out and add dirt on the bottom and keep the seedlings in a quarter pot until they are at least a foot tall. Then spring time of following year as they start waking up from dormancy I would put them in a gallon pot. And sometimes they stay there for a long time.
What I noticed is they get stunted If they are not repotted right away and Seedlings won't bloom in small pots.