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I don't have any timelimit that I follow, some mixes are way shorter and some are much longer. I keep adding new tracks as long as I find good ones to use, that fit well and that I think will make the mix better. So far, my shortest mix is 3 min and 48 second and my longest 43 min and 28 second. :)

But in general, they tend to be between 10-15 minutes, more often than not.

@freakygeak