Your child's body will decide when it's time.
A dry nappy is a good start
On average, it takes ten months from the point when kids can stay dry during the day until they can do the same at night. But it can also take much less or much more time depending on the child.
If your child's nappy is dry several nights in a row, that's a sign that the time is right. Use a mattress protector on the bed, and prepare for accidents. If your child wets him or herself, just start the nappies again. Nappies are available for children of all ages.
Should I wake my child to go to the toilet?
Many parents wake their children and take them to the toilet as they themselves are going to bed. This isn't really necessary, since the point is that your child should learn to wake up when they need the toilet, or get used to not weeing at night.
But in many families this is seen as an appropriate preventive measure. If it works, that's great. Most children learn to stay dry at night some time around nursery age. Contact your GP if your child continues to wet the bed and you are concerned.
Read the article on potty training and giving up nappies.
Dry nappy all night long
The ability to stay dry is hormone-related and develops over time. There's no need to try and get your child to practise not weeing at night if their body isn't ready yet.