In basic terms (according to this document, the low res camera and other sensors on MGS can map the entire surface every 7.2 days.

MGS is in a sun synchronous orbit around Mars. Currently, the camera is aimed 16 degrees off of vertical so the images are slightly oblique. This allows things that were imaged before to be reimaged at a slightly different angle.

The high resolution camera images are really narrow, only a few kilometers wide. It's like looking at the surface through a soda straw. It's possible to reaim the camera take an image that wouldn't normally appear in the camera's field of view. However, that would interfere with other scientific images. Given the very narrow field of view, it's also a hit or miss proposition because you could be off your aim by a fraction of a degree or by a few seconds and completely miss the desired target.

Being in a sun synchronous orbit (nearly a polar orbit), the north and south polar regions would probably be much easier to revisit than places at lower latitudes.