My understanding is that there is an optimum moisture level in the bales, and that it can be tested fairly accurately with a moisture sensor that is pushed into the bale (to test toward the center of the bale). Alternatively, but less accurately, it can simply be weighed. The bulk of the weight in a bale is its moisture content. If you know the given size of the bale, and it was baled under a pretty standard compression, weighing the bale will give you an indication of its moisture content.
I think, if you're getting started, you'll want to use a moisture sensor. Here's an article about it:
http://www.strawbale.com/how-to-choose-the-right-straw-bales/
As for how long to wait before baling-- I have no clue. I assume that people are using bales that are made in a typical fashion, so however farmers would typically do the baling (back when baling in the rectangular bales was common).
I would also assume that the
dryer the straw is before baling, the less time it will take for the bale to dry to the right moisture level. But I think that straw is usually baled at a higher level than construction would warrant, and some drying time as a bale is recommended. Of
course, bales sitting in a warm, dry barn loft will probably dry faster than a stack sitting between two tarps on the ground somewhere.
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