They did not have the map for more than two years while Ron was in Hogwarts the twin gave it to Harry by year three who promptly lost it to Lupin. In fact the twin also explain to Harry that they know it by heart. If you know a map by heart do you feel the need to use it as often or at all? The "rat" use to belong to Percy but I don't think JK ever says when exactly he gave it to Ron. It just a plot tool, you're not supposed to study it too hard anyway.
from overview for madhi19 http://ift.tt/1p3DseJ