A short answer to your question. When you wet the leather for the first time, wet the back then wet the front and pay attention to how quickly the leather soaks up the water. I wet it let is soak in, then wet it again (just the front, I only wet the back once). Usually the second coat of water is enough. I can tell because the leather doesn't soak up water as fast.
If you really want to do it right, after you wet it, slip the piece in a ziplock overnight. this will let all the fibers swell with water, and you can literally stamp for hours on it.
by the way I use a sponge.
As far as when to re-wet it, (if you don't use the ziplock method), Re wet when the leather feels warm to the touch. I use the back of my hand to touch it. If it's cool, keep stamping if it's warm, wet it.
This does come with experience though.