Between the ages of 5 and 6 years old is when most kids can take a bath by themselves without supervision.
Even when a kid is 6 years old and even 5 years old they need partial supervision and help setting the bath water to the right temperature.
However when the child is at least 5 years old you can usually safely leave the child alone in the bathtub for a few minutes but do come in and check on them.
When the child is above 6 years old then they can take a bath by themselves completely and do not need any supervision.
But I would still check on them every 10 minutes or so just to make sure they are safe and if you hear no sound from them holler for them and go check on them if no response.
Never leave a child under 5 years old in the bathtub alone for even a second and especially if they are under 4 years old as they can easily drown.