Most magic that isn't instantaneous is permanent. Disillusioning and similar spells (usually used in the creation of invisibility cloaks) is the exception - these spells have to constantly adjust and interact with living creatures. A spell that, for example, expands the inside of a Muggle car is comparatively simple - the car simply has more room inside.
It seems that, even with magic, nothing is free - the power it takes to camouflage someone who is under one of the invisibility spells seems to degrade the spell over time. Similarly, an invisibility cloak that is struck by a curse can be damaged, and the damaged area no longer provides invisibility.
Harry's cloak is special, being a Hallow. The enchantment placed upon it is very powerful, making the cloak invisible effectively forever, and it seems to be resistant to at least some spells. I doubt that it would resist if any of the serious wizards had tried to damage it, but it is obviously resistant to simple wear & tear.
The Marauder's Map, however, does get damaged. It is showing wear and tear, and looks very raggedy. It's also considerably less complex than an invisibility cloak - it simply maps the insides and grounds of the castle (not easy, since it's unplottable, but not a difficult piece of magic) and displays the locations and names of the inhabitants. It's just speculation, but I would assume the map simply relays the information, which is gathered by a spell cast over the castle & grounds itself (James & crew could have done this intentionally, so that their spell would be refreshed whenever the Unplotting spell was). I'd be interested in seeing if the map would work if you were a significant distance from the castle.
So, in short, the Map is completely unrelated to the Hallows, excepting in that they are both rather cleverly constructed. And there's no more reason that the map's magic should 'wear off' than there is reason for the Weasley's car to suddenly lose the extra space.