How often do you like to see the beverage cart during a round?
For me, every 6 holes is plenty. If I have a Gatorade on hole 1, I don't think it will be gone by hole 2, so every 6 holes would be great. You see them 3 times a round and in between you can grab something at the turn if you need it.
Also, that keeps you from being distracted and it should allow the pace of play to stay steady throughout everyone's round and not slow groups down because they feel they need a new drink every single hole.