In children eight years and younger glasses should be worn full-time unless you have been advised otherwise by your ophthalmologist or optometrist. This is because there is a critical period for visual development. If refractive errors are not corrected, or compliance with the treatment is poor, then a permanent reduction in vision may occur. Glasses may be removed at bath time and bedtime.
This depends on many things including the strength of the glasses, the age of your child and whether they have a squint. Vision develops until the age of 7, so it is essential that your child wears their glasses during this developmental period. If your child does not develop normal vision by this age it is very unlikely that they will have any further improvement in the future, which may result in permanent problems with their vision including the ability to drive. The Orthoptist, Optometrist or Ophthalmologist will discuss the long-term need for glasses with you.
It depends on how often she or he loses things. When I was younger I would only wear my glasses when I needed them but then I would lose them or forget to put them on. If she doesn't lose things that often let her decide, just make sure she wears them to school. I don't think not wearing her glasses all the time will make her eye sight worse.