I clamp till I need the clamps or 30 minutes, whichever is longer. If you are faced with a 30 minute scenario, I make sure I have somewhere for the items to go before I pull the clamps, then place the items in that spot and leave them alone till tomorrow. YMMV.
From Fine Woodworking Magazine:
Now long should the joint be subjected to clamp pressure? The time varies from species to species, with woods that have an even density across the growth rings, such as maple, requiring less time. But in general, the glueline reaches around 80% of its ultimate strength after 60 minutes of clamping. After this, joints can be released from the clamps, but the full glue strength won’t develop for about 24 hours.
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