Changing Gravel, How Long to wait.

Unless your main filter is UGF/RUGF OR your filter can hardly handle the bioload OR your tank is not really mature yet, you can change all gravel in one shot.

With UGFs/RUGFs, marginally effective biofilters or in a case of a relatively new tank, I'd wait for, at least, about 3-4 weeks.

Naturally, you can always play it safe if you wish. Less room for an error.

Ditto.

If you have any doubts, feed smaller than normal meals for the first week after or skip feeding entirely. A reduction in bacteria bed can be dealt with via a reduction in waste production and your fingers directly control what comes out of the fish since they provide what goes in.