This depends on how much you’re singing and how often.
If you’re singing every day, then rest your voice in between for 10 to 20 minutes depending on how hard you worked your voice and how tired your vocal muscles are. You can practice for 20 to 40 minutes and rest for 10 to 20 minutes, and then practice again.
How long you sing and rest varies. With time you will know what is best for your voice and style of singing. This is best done with the guidance of an experienced coach. Always remember to warm your vocal muscles before singing songs.