You need to test immediately before you eat and then 2 hours after first bite. The rise from before to after will show you what that meal has done to her levels, which gives her the opportunity to change that meal in some way if the rise is too great.
There are no set rules. You can also test an hour after, and 90 minutes after, and 3 hours after, or whenever you like, plus first thing in the morning after fasting all night, and bedtimes. It is up to you. However, if you want to learn from it, you need to keep a food diary including portion sizes and record these levels alongside so you can study the reactions and watch for patterns emerging.