First thing is that you haven't shocked the pool! You may have surprised it a little bit, but you're a long way from shocking it. Shocking is a process that must be maintained until you meet the criteria for complete. (1) 1ppm or less FC overnight. (2) .5ppm or less CC. (3) Your water is sparkly if you don't have a FAS-DPD test. It's not a product that you throw in the water once.

Instead of how often can a pool be shocked, you should ask how do I correctly shock my pool? If you maintain your FC above the recommended level for your CYA you'll never have to shock your pool. Now, if your pool "needs" shocking you need to shock completely and you won't have to do it but once. To directly answer your question, as many times as it takes, but remember throwing a product in it isn't shocking it so for instance throwing a bag of Dichlor or Cal Hypo in every day isn't shocking the pool every day.

Get a good test kit and you will know if you need to shock or not and you'll be able to shock correctly if you do.