Hi Phanhtho! There is no exact time that has to be followed. It's best to listen to your own body. Generally speaking I always suggest at least one day off a week. I also suggest at least one week off for every month of taxing and tough workouts. If you are training three days a week, your body has 4 days recovery built in to every week so you might not need a full week recovery after a certain period of time. The best way to know is to listen to your body. Signs of needing a few days to a week off are feeling tired and achey all the time, having insomnia, being in a crabby mood all the time, getting a lot of colds/fevers, feeling a lot of pulls and twinges, dreading your workouts but feeling you have to do them out of guilt, and hitting plateus and seeing no improvement.
I hope this helps