When are grafts stable enough to use full shower pressure and regular washing? When can I comb my hair after FUE?

It is best to ask your surgeon about post operative care as each doctor has their unique regimen. In general you can wash normally after two weeks.  Many doctors have their own rules.

By 7-9 days post procedure the grafts are firmly incorporated into the skin and will not be dislodged by the pressure of a shower.

You have to wash your hair gently for the first five days after your hair transplant. After day 5 you can wash it normally and shampoo and comb without the risk of hair loss. Here is a guideline of step by step care of your hair after a FUE procedure. 

Grafts are stable one seek after all of the crusting is off. If you had any post operatieve crusting, then your post operative routine was not good enough.