As long as it is a permenant crown, not a temporary, it is pretty durable. That is as long as you have a good dentist, who does good work. The only thing is that, on the porcelain/white crowns, hard candies like suckers can get stuck to them. I got an astro pop stuck to mine once. If in this case don't try to pull it off, just try to break it off in your mouth, and suck on it until it dissolves. Sometimes as the crown gets older they do pop off, in that case, as long as there is no decay, you will just need to get it glued back on. Regular flossing shouldn't take it off, if it does, then you should see your dentist and have him fix it so that, that won't happen anymore.
Good luck!
p.s. I didn't lose my crown with the astro pop, but as much as I panicked and tried pulling it off, my crown never came loose. But I won't dare try that again.
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Can crowns fall out when flossing? 
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