How long do I leave the basking light on for my baby red eared slider?

Mrs. Frankenstein

You can do the 12/12 rule- 12 hours on and 12 hours off. You can also go by the natural light you get in whatever part of the world you're in, so sun up ON, sun down OFF.

oikoσ

Mrs F is right. Go by the natural light if you are planning to breed the turtle.