I would like to add that the lineage you are dealing with has a lot to do with this. I started out w/ bigger labs-- and when LBP food first came out, I kept those those pups on it longer (~18 mos if I remember right). BUT, I wasn't doing as much w/ my dogs then either (to need more calories).
Now I've got "little" labs.... famales topping out at 60# and males averaging 75 (assuming the owners don't let them get fat... that's another story). So for my small, slow, even growing labs that are pretty well strung together by 1 yr, that seems to be a fine time to switch. If I had a Dane, Mastiff or St Bernard (or oversized Lab even), 2 yrs may well be the ticket as a large dog isn't well tied together yet until about that age. Interestingly enough there was a question on another lab forum recently polling weights--- I can't believe the number of 100+ lb labs out there.