I'd say start reading bedtime stories when the house is quiet and the child is ready to wind down for the night. Often, that's around 8 PM. It helps them calm their minds and have a peaceful sleep.
Well, a good time to start could be when the child has finished their bath and is relaxed. Maybe around 30 minutes before their usual bedtime, so say around 7:30 PM. This gives them time to unwind and get into a sleepy mood.
I think it depends on the child's schedule. Generally, starting around 7 or 8 PM can be good.