When you use toner in your hair you can leave the toner in your hair for as long as 30 minutes but you should not leave the toner in your hair longer than those 30 minutes.
Usually 20 minutes of the toner in your hair is good enough but you can leave the toner in your hair for the full 30 minutes if you want too.
Leaving toner in your hair longer than 30 minutes could damage your hair and do more harm than good so remove the toner from your hair after 30 minutes to avoid damage to your hair.
Also some hair toners that you buy have to be mixed with a peroxide activator for the hair toner to work properly and that too can damage your hair if left in your hair longer than 30 minutes at a time.