Balloons tend to spawn at every 4th and 9th minute (e.g. 9:04, 9:09, 9:14, 9:19, etc.) They float from left to right and switch to floating from the opposite direction after sundown (between 6-8 PM from what I've noticed on my island). I go into a building one minute before a balloon could spawn and come back out. Listen for a whooshing sound as you run up and down your beaches. Hope this helps and good luck!