Taking breaks from a large project is the best thing to regain interest.
I have maps that I've gone weeks without even touching, sometimes I open them up and just "walk around" to try and get back in the mood, sometimes it works, sometimes it doesn't.
I think the best thing to do is find a project that really excites you, something that you want to be able to play sooner or later.
And take your time, a friend of mine has been working on a map for over a year now, he has had it in a releasable state for at least half that time, but he insists on adding, changing, tweaking and just generally making it a better map.
I suggest you spend no less than 1 hour on each corridor, room, small section of your map, make it so damn good that you can't give up.