So what was the question? I can't tell from here what stage the pile of horse manure is.

So I'll guess that the question is which stage is best for the worm bin? The worms themselves probably wouldn't mind being right in the pile but when you go mixing fresh manure with carbon material like bedding you could have the bin warm up so always make sure you keep a "safe" zone in your bin where there is not food to cause it to heat up.

However, another thing you might wish to keep in mind. Warm blooded animals can have pathogens in their manure... so if you wish to spray worm tea from a worm bin that gets manure on your plants that you might eat without through cooking, then you should really make sure the manure is well aged before putting it in the bin. Yes, I know many people think that worms take care of any pathogens that pass through their bodies but how do you guarantee that every last speck in the bin has actually been completely processed and has not touched unprocessed stuff in the bin? So, if you intend to use your worm castings on and around food plants during the plant's growing season, it is probably best to stick with the composted manure. If you only use worm castings when preparing planting beds before planting, then it won't be as as big a deal what kind of manure you use. Also, if you make sure to age your castings before use, then that can take care of the time to harvest issue.

If you are asking how long the manure should age, give it about a month. If the pile is being turned to accelerate composting, two weeks should be enough. If the horses have been de-wormed, give it 6 months or more. (Someone please chime in if 6 months is not long enough.)