The flooring that we use is called "Portafloor"  and is manufactured out of a plastic type materiel.  It has holes in it to allow moisture and air to get to the grass.
The longest time that we have left it down has been two weeks,  when it was removed the only difference in the grass was that where the floor had been there was no pine straw.

In the picture below the flooring is over plywood that is part of the pool decking system,  under that floor is the swimming pool.