The Canadian Paediatrics Society recommends the following amount of sleep in a 24-hour period.  Keep in mind that this is just an average and some children need more, and some need less.

Sleep Needs by Age

 From Newborn to about 4 or 5 months old, sleep is very disorganized and your child will sleep when they’re tired, rather than on a consistent schedule on the clock.  It’s important that baby is napped very frequently, with only about 1 to 2 hours of wakefulness between naps. There won’t be a specific bedtime either, but you will notice that as baby gets older, his bedtime will naturally start to get earlier.

When your child turns 4 or 5 months old, their internal biological clock or circadian rhythms will start to form, which means they are ready for a more predictable nap schedule on the clock.  A consistent morning and afternoon nap will emerge, with an optional catnap in the late afternoon.  Bedtime should be about 4 hours after their second nap ends. If your child wakes from his afternoon nap around 3, bedtime should be about 7pm.

You can also use the table above to help you determine what bedtime should be. If your child is 18 months old and naps for 3 hours during the day, they should get about 10-11 hours at night. Work backwards from the time they wake in the morning, and set that as their bedtime.  Typically a 7pm bedtime, or even earlier, works well for toddlers and preschoolers.