Usually between 6 to 8 months.

 They usually come into 1st heat around 6-8 months. Most people like their pet to have the 1st heat, and then get them fixed afterwards. It helps to keep her healthy and aware of her instincts.
They aren't good at being cooped up, they need a lot of exercise as they are bred for labor and sledding, so they are naturally active, and can be somewhat destructive if left alone in the house for long periods of time. Hope this helps, good luck.