My project consists of maple plywood with maple veneer edge banding. I plan on using General Finishes waterborne stain, followed by a waterborne polyurethane outdoor varnish.
How long do I need to wait between applying the stain and the first coat of polyurethane?
The instructions on the General Finishes website are generally helpful and verbose, but don't contain this key bit of information.
Dry and cure times for water based products are generally shorter than their solvent based alternatives (that is not universally true but should apply in this case).
The general answer to your question is that the longer you wait the better but 24 hours between the stain and the sealer should be plenty of time. If you are in a hurry for some reason you can wait until the stain is dry to the touch (meaning you don't get color residue on your fingers if you wipe your fingers across) and begin sealing.
In the past I have wiped stain on, let it sit for about 30 minutes, wiped it off and immediately applied my topcoat.
It has always worked fine for me but I would not recommend it for a high quality piece of woodworking, nor for anything that will get a lot of usage. So no chairs, tables, cabinets etc.
How long do I need to wait between applying the stain and the first coat of polyurethane?
It's impossible to tell you for sure how long to wait because of all the possible variations, including: the moisture content of the wood, the amount of stain applied and/or left on the wood, the local temperature and humidity of course, as well as the amount of airflow.
In most situations waiting some hours would be enough but waiting longer can't hurt. Recoat time for some brands of waterbased wood stains is stated as two hours so that gives some idea of the bare minimum drying period (remembering this would be for a typical application and average conditions).
Err on the side of caution and wait a few hours at least, a full day if patience allows.
