Dependant on age is how much they need.
They need less as they get older.
Figure out what time they have to be up, go back from that, that's bed time.
Newborn 0-3 months old 14-17 hours
Infant 4-11 months old 12-15 hours
Toddler 1-2 years old 11-14 hours
Preschool 3-5 years old 10-13 hours
School-age 6-13 years old 9-11 hours
So someone of school age between 6 and 13 that had to be up at 8 am to get ready and catch bus or be dropped at child minder etc would need 9-11 hours, so say 10hours. 8am minus 10hours, 10pm.
17 month old is 1-2 toddler, needs 11-14 hours.
If you're up at 8, that's bedtime as late as 9pm.