The time it takes can vary depending on your connection, but generally speaking it takes 5 minutes before a Poke Stop refreshes. If you find a bench nearby and are low on Pokeballs you can chill out and keep taking items from the stop ever 5 minutes once it refreshes.
The content that gets dropped from the stop depends entirely on your level. The higher level your pokemon trainer is the better the items that will drop from the Poke stops.
For more information on Pokemon Go, check out our Pokemon Go Wiki.