Is it good to leave conditioner in your hair overnight

And it is absolutely fine if you want to leave the conditioner in your hair for long or overnight. Some conditioners, especially leave-in conditioners, are meant to be left in your hair, while you should wash rinse-off conditioners after a few minutes.

Leave it on for the amount of time specified on the product label, from 10 to 30 minutes. Rinse out the conditioner.