You can keep mated queens in nucs indefinitely, I often do. I use 5-frame medium depth nucs and in the Summer I stack them two high, a total of ten frames. Once they fill up all ten frames, I simply harvest a few frames from several nucs to make up another nuc, replacing the combs with new PF120 frames or foundationless frames. That keeps them busy for another little while, before I have to do it all over again. Sometimes I use the combs of brood, pollen, or honey to help boost production hives early in the season - this can work miracles.
I also run a few, what I call mating condo's, I've heard them called queen castles, though mine are only medium depth. They are a normal 10-frame super with partitions (its amazing how thin you can cut strips of 2x4 with a band saw, then stack them and glue them in as partitions) and drill individual entrance holes for four 2-frame compartments or three 3-frame compartments. I use them as advertised, to mate queens, but they can also be a starter core for a normal sized nuc colony. I make them up with a comb of emerging brood and a comb that is empty on one side and full of sugar syrup on the other side (two of these, if 3-frame). The frame of emerging brood quickly occupies the compartment, the queen emerges, mates, then fills every empty cell with an egg, sometimes more. I use the nucs and queens I produce to keep my production hives going strong and sell any extras.