The exact time depends on a bunch of things: what types of food you add, what type of bedding you use, moisture and temp levels in your worm bin, other stuff up to but not (necessarily) including the phase of the moon.
I change my worm bin bedding when it no longer looks like what it started out as. That is, if I've used corrugated cardboard for bedding, and there's a bunch of soggy brown-paperish stuff and lovely black castings where the cardboard used to be, I add more cardboard. I also sift out the castings and return the soggy brown-paperish stuff and as many worms and worm cocoons as I easily can to the bin.
My guess for your bin would be somewhere in the neighborhood of 6-12 weeks, give or take.
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