I'm going to have to do some experimenting and temperature checking this weekend. Today it took until 3 o'clock to get to 60°, and then I only stayed there for about an hour. I would be happy if I could get three weeks on either end of the season. An extra two months would be great. I will always bring my tortoise in during the winter.We have long tedious winters and the ground takes a long time to warm up. In the spring, I don't leave them out overnight until the night time temps are 60 degrees. That's their indoor night temp all winter, so it's no big deal to acclimate. By then the ground had dried & warmed.
Weather predictions may be off a bit, and that leaves a safe margin.
As the season progresses, I will leave them out at lower temps, but 45 is my cut off. If it's cold and rainy I'll bring the tortoises in. Covering the habitat works if I'm there to make sure it won't overheat if the sun comes out.
A green house solves that problem.