I was even thinking of using masking tape to hold the piece in place since, after all, you dont want to squeeze all the thickened epoxy out anyway.

   I was even thinking of using masking tape to hold the piece in place since, after all, you dont want to squeeze all the thickened epoxy out anyway.

The best rule of thumb I can give is always let epoxy cure hard before removing clamps. Like you said this depends a lot on temperature but also on the epoxy being used.   

Another consideration that can be important for best adhesion: setup your glue to cure while the temperature of the wood is falling, that way air (and epoxy) is drawn into the wood you're gluing. In my garage this means the best time to glue is afternoon and let it cure overnight.

An alternative clamping method for this application might be screws and plywood pads. The screws put pressure on the pad which puts pressure on the spacer to be glued. The screws need to be oiled beforehand, and excess oil then wiped off, prior to gluing/screwing, and they must be taken out the following morning to prevent breakage. Before trying a new technique like this, it's best to do a small trial to make sure it will work for you when it counts.

   If you are not bending the wood into place, that is, no stress in the joint, then you should be able to remove the clamp as soon as the glue is set (gelled).  As tmh2111 says, even tape will hold things in place if there is no stress in the joint.  Your spacer blocks should fit this description.  If you are laminating or installing bent pieces, defintely allow a hard cure before unclamping, for example, the inwale that will follow the spacer blocks.