How long to wait after tile installation before placing heavy objects on it?
It depends on the concrete permeability and the tile permeability  (is it porcelain (essentially zero permeability) or a ceramic bisque (most non-porcelain tiles))?

A sealed concrete floor and a porcelain tile will greatly extend the cure cycle by limiting moisture migration.

It is for this reason that Schluter (a tile membrane manufacturer) specifies a non-modified (no latex added) thinset for its floor membranes and porcelain tiles together (on the tile side of the membrane).  They make the same requirement on the floor side, if the floor is limited in permeability.  Flexbond is VERY modified.   

If sealed floor and porcelain, 48-72 hours is safe.  The tiles will cure properly with EVEN compression (the weight of the washer), if spread over several tiles.  

Would it be possible to temporarily put down several 1/2" ply feet, that span from front to back, under the washer/dryer feet?