How much time does it have to pass, for a PokéStop to be available again?
Official support page doesn't mention it.
PokeStops will be a purple-ish color while they are cooling down, and they will restore to their original blue after their cooldown period has finished. This takes about five minutes from your last use. Other players do not affect your use cooldowns.
You may "hack" PokeStops for five minutes on end, as they do not burn out and become unavailable.
5 minutes. 40 meters.
Purple = Not-active
Blue = Active