How long do you have to wear a retainer after you have your braces taken off?

I have had braces for 6 months now and my dentist said I'll have them off in another 6 months (he also said that I had the quickest correction of teeth he's ever seen) so I'm wondering how long I'll have to wear a retainer after I have them taken off.
I have them on upper and lower teeth too.
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