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.