After you visit a pokestop and the icon on the map turns purple, you will no longer be able to use it again until it turns blue again. How long does it take for a Pokestop to refresh?
It usually takes 5 minutes for a stop to refresh, but it can get a bit delayed at times due to some networking or server issues. A few seconds before the stop is going to be available again the color will gradually begin to fade from purple to a lighter blue. The color will eventually turn back to a dark blue, but will not be immediately available. It can take up to 30 seconds after the stop turns blue again before it can actually be spun to collect the rewards again.