It can only be eaten 3 times a day, and only when your thermometer has reached Inferno status. This counter (and the temperature bar) will reset at 00:00 game time. If you are logged in at this time, you have to logout first for the counter to reset.
During a heatwave (in the weekends) you can continue training even when your temperature gauge has reached Inferno status. However, resetting the gauge does still count towards unlocking several rewards. If you haven't unlocked the rewards from the previous years, those will start first, starting at the oldest. You can Right-click a Lifeguard to view your "Beach Stats", or Reyna to "Check Title" progress.
During the Happy Hour Spotlight Plot (which actually lasts a full hour and not 15 minutes) your temperture bar will not increase, you will also receive a 10% experience bonus on all skilling activities.
It takes 300 successful skilling actions, 500 Tropical coconuts, 94 Fish caught, or 108 Fish preparing/cooking actions (i.e. 54 clicks on the chopping board or grill, you gain experience twice per click) for the weather condition bar to go from 0% to 100% (excluding those done during Happy Hour ofcourse). Thus, if your goal is to get the most experience possible out of your temperature bar, you should maximize the experience obtained per action. You can do this by selecting the 'Lower chance, Higher XP' option where available, and use boosts whenever you can. Depositing Tropical trout (on the fish table near the cooking activity) or Tropical coconuts (at the Coconut shy) doesn't cause the temperature to rise.
If you are only after obtaining the tokens as quickly as possible, it makes sense to select 'Higher chance, Lower XP' (where applicable) to maximize the number of successfull actions.