The instructions on the package do not say anything about waiting for 20 minutes or the like. That said, when mixed in the proportions stated on the package, the glue is a bit runny. It does thicken up as it sits, but I find that if I apply the glue without rushing around (take my time), by the time I am done, it has thickened enough for me to go back over and apply a little more just before the clamps go on.
Long winded to say, yeah, a little sitting around will do your glue pot some good.
Russ