It is quite variable - lots depends on your personal breeding schedule and how long a break you want over the winter as well as how soon you want to get them going in the spring to have early babies and beat the rush of hatchlings on the market.
With corns, that will breed with good fertility without any brumation, there may not even be a minimum time. We have had good fertility and breeding success with some that were cooled for only 6 weeks, but usually keep them napping for almost 3 months for convenience and because we aren't in a rush to get the girls bred in the spring. I find the real advantage of brumation is that it allows us to coordinate the timimg of breeding.
So in my opinion, the duration really depends on what you want to do -
mary v.