The Landing Checklist will have items on it: Gear........and end with Landing Flaps........ Therefore there is no point in starting it too early. On the 737, you should go 'Gear down, Flap 15, Landing checklist to the Flaps' and hold on the Landing checklist until you are ready for final flap, then complete the checklist from the checklist- do not put it away until the checklist is complete. Sometimes you may not be taking final flap until about the 1000' mark. I am not aware of an external limit to start or complete the checklist, the aim of the game is to get it completed by 1000'.