On average it takes a group of 4 people around 2 hours to complete 18 holes ( e.g. 6-7 Mins per hole).
There is a limit of 5 players/balls on a fairway at a time, so dependant upon the size of your group your overall visit time will be longer, i.e. your group will split into smaller groups of 5 and play each hole one after the other.
So if you had a group of 20 players in four groups this would add approx another 25-30 mins.