1. There is No question of invalidity of Prayers just because of not shaving the unwanted hair.
2. No doubt, shaving the unwanted hair is highly recommended as it makes the body clean and avoids the harms of the dirt which accumulates because of the unwanted hair.
3. Removing the unwanted hair starts when ever such hair grows, and it is to be repeated whenever it grows again.
4. It is recommended not leave it unremoved from the body for forty days or whenever it grows up.