Yet another post that is going to get limited response. Where does one begin with a post like this?
There are so many fine points to canine conditioning, and here we have a person that just wants to know
"how long they can leave a dog on a mill", as if everything is equal amongst all dogs, all e-mill speeds etc. It doesn't work that way friend! You need to build up some fundamental knowledge of canine conditioning,
THEN use specific training devices to accomplish your intended objectives.
You can have a Top Fuel dragster that will go 330 mph, but if you don't know how to start it, tune it, or drive it, the car itself is pretty damned useless. Likewise with conditioning tools, if you don't know what you are trying to accomplish with the tool, how to use it, how to read a dog, etc., having an e-mill; or any other device, is utterly useless. Buy
The Pit Bull Bible for starters, or if you have bought it please read it.