If still with mum, so long as they have access to plenty of good grass, you don't need to cake them at all, they'll grow faster with, taste sweeter without.
If they're pet / orphan lambs, I usually keep them on half-a-pound to one pound per head per day of a lamb cake (not creep, I only use that to get them started on eating cake) throughout - they won't ever grow as well as naturally-reared ones and need the help. But they would eventually grow on and finish on just grass, I guess, if you wanted to stop the cake. I think I would tend to keep them caked until 4 months old, though, which would be a natural weaning age.