As a rule of thumb a lesson will last for an hour.
Younger children or those with attention difficulties may just for 45 minutes. That is down to the discretion of the parents.
Older children (GSCE age) or adults, especially those with exams on the horizon or studying 2 subjects may choose to have a slightly longer lesson.
This takes careful consideration though as to concentrate for more than an hour is exhausting especially if it follows a full day at work or school.
Sometimes it is more effective to have 2 short lessons than 1 long lesson.
Again this is at the discretion of the family.