How long do you usually have a crush on someone before it wears off?

Also, how long would you have to have a crush on a person before you cracked and finally did something about it?

I'm trying to find out if a guy who answered a Facebook app question about having a crush on me still feels the same, or seeing how I can tell. Thanks for your help!
How long do you usually have a crush on someone before it wears off?
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