Noon mostly. Quite a few people are up in time for the bars to open. But it normally gets rowdy in the early afternoon.
Afternoon? that's a bit mental! are the clubs/bars in one area or in spread out in terms of location?
The important ones are within a block or two of each other, with Senor Frogs really close. Basic is further South.
Remember clubs are all you can drink, so its not gonna cost you anymore the earlier or later you go... just make sure you tip good, early!
