Just depends on if you are going with children and if you know what you are looking for. There is plenty of selection. As others have mentioned 20 minutes is a fair amount of time to spend in the store. The discovery center is much more hands on and really let's the kids play. This can easily last 1-2 hours.
20 min for the store, 1 to 2 hours for the discovery center if with kids.
Average time in the place I would say 20-30 minutes for the smaller retail location. LEGO Discovery portion 1-2 hours but it is always busy.
What all does the Lego Discovery Center offer?
If you're in a hurry at the store ask for help/suggestions from the staff. I've done this and they're genuinely very helpful. It's of course a lot of fun to browse for a long time as well, if you're so inclined...
20 min tops
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