How often do Snorlax appear in Pokemon X/Y?

With help of people in the thread, I tried a few things. Just waiting around will not make him reappear.

What will make Snorlax reappear in the bridge is to beat Elite 4. If you want it to appear again, simply go to Pokemon League and re beat the Elite 4.

This will apparently also work with other Pokemons like Mewtwo and Zapdos if you failed to catch them (thanks, scenia).

There are two ways to catch Snorlax (both on the bridge on route 7). The first is when you have to go to Parfum Palace and get the Poke flute. But after you have defeated the Elite-Four (includes times after your first completion of the game), he will reappear on the bridge and go to sleep. When you revisit this area, you will be able to battle him as you did the first time.

That snorlax will appear on the bridge until you catch him, after beating the elite 4 he will reappear anyway. My friend noticed this with his game too. I thought it was weird since I caught it the first time around. So just wait a day and try it again until you capture it, or set the date on your ds to a different day to do it faster.