Each mill and each dog's approach to the mill is so different that one cannot give a proper response to the question. Monitor the dog, and help him progress incrementally.
I have written numerous pieces on conditioning in the past and do not feel to do so now. Remember the paces of a show and that will guide your training. The paces are PUSH (simulated by resistance and strength training), COAST (simulated moderate paced roadwork, millwork, and other means of aerobic training such as swimming....if the dog likes it) RECOVER (simulated by rest in the workout), and EXPLODE (which is simulated by fast twitch speed training such as flirtpole work).
That is a simplifed explanation of a complex very process. If the work is too one dimensional and does not hit all of these paces, it will be exposed in a show at some point if other factors at play are reasonably equal.