You can test whenever you like, but to learn what foods do to your levels and which foods are best for you personally, you need to test immediately before eating and then again 2 hours after first bite. It is the rise from before to after that is important, and should always be under 2mmol/l and preferably less than 1.5mmol/l. If it is above 2mmol/ there are too many carbs in that meal.
It is best to keep a food diary including portion sizes and record your levels alongside. Then after a period of time you will see patterns emerging that will tell you where you are going wrong with your food choices, and also what you are getting right.
You can also test at bedtime and again as soon as you get up (before doing anything else apart from washing hands) These readings will tell you if you are having over night liver dumps (dawn phenomenon). The morning test is known as FBG (fasting blood glucose)
Once you get into the swing of things and begin to understand what is happening, you can test more frequently after meals, such as an hour or 90 minutes after first bite, or keep testing after the 2 hour mark to see how long it takes you to come back down.