No right or wrong answer. It depends first on the dog. The better conditioned the longer he can stay, regardless of the mill. Secondly, it depends on the mill. The free-er it turns, the smoother it turns the longer a conditioned dog can stay. One is somewhat dependent on the other.
In the video I posted, on a mill that turns like that I get about 10-15 minutes per session, granted my session may last an hour, on and off, on and off. In straight all out runs anywhere from less than a minutes to as much as three to five. Depends on the dog and where we are at in his progress.
With the carpet mill I see more advantages to stopping and starting to an all out sprint, stopping and starting to an all out sprint. I am more concerned with how long it takes him to recover than how long he actually ran the mill. Granted his run times will increase in time but the recovery times speak louder to me. If that makes sense.
Not to be short with an answer, but there is no 'time' a dog can safely do anything without knowing the dog and the equipment being used. My minute may be your ten minutes or vice versa. Not knowing your dogs or your equipment I can't say 'X' amount of minutes is safe and 'XXX' amount is not. EWO