I repot a plant whenever I think it needs it. I grow many plants in the ground, but those in pots, if the roots start coming out the bottom, I repot. For me, its better to go ahead while I see the problem than try and wait because I will forget....I have about 1000 plants.
I also repot things that I buy that I know are not in the proper substrate for their species when I get them. If I was to buy a Hoya and it was potted in a peat based heavy media like you get from Costa Farms I would immediately repot that into my own substrate mix for epiphytes because I know how I water, and I would overwater and kill that hoya in about 2 minutes.
I do the same thing with other epiphytes....monsteras (not deliciosa...I collect other more unusual species of monstera), philodendrons, anthuriums, anything I get that I feel is in a substandard substrate.
I repot when a plant needs to be 'stepped up' because it is too large or heavy for the present size of container.
I don;' hold with the theory that gets bandied around on this site that repotting is somehow bad for the plant or its root system. I think that's hooey. I repot all the time, and my plants never suffer any setback of any kind.