Beetroot can be harvested anytime during the growing season, including leaving the beetroot in the ground until the first frosts.
However, having said that, you can leave beetroot too long in the ground which results in beetroot to then become woody and dry.
The best size is about 50 days after planting the seed when the roots are about 2 inches in diameter. A full sized beetroot is 3 inches in diameter, but it is better to eat them before they get to this size.
Anything after that and they become fibrous and thereafter woody and even bitter.