Especially at first, I would try testing once a day but at different times each day. For example: fasting, two hours after breakfast, 2 after lunch, 2 after dinner. That way you can get a sampling of where you're at different times of day. Also testing two hours after meals is a good way of seeing how different foods affect your blood glucose and knowing then what is good and not so good for you to eat. So bottom line is there is no absolute time you have to test but it does provide you with important information about managing your diabetes.
There are different schools of thought. My two-cents worth is that initially, you should be testing very intensively. Later on, when you get a handle on your insulin utilization situation, you can test less often.
The problem is that none of are very good at "guessing" our blood sugar levels..
(1) Fasting (or one definition anyway) is the level when you get up in the morning without eating all night.
(2) Generally, two hours after every meal.
(3) Before going to bed.
There is a caveat. You should eat "measured" meals, of known caloric and nutritional value. If you are going to gobble 500 calories one day, 2400 the next and 1100 the third meal, the drill becomes pointless.
Some people keep a notebook detailing their meals and their beverages. Be very careful about drinking apple or orange juice, which can push sugar levels up very rapidly.
After a while, you will know how much insulin or metformin or exercise is required to maintain normal glucose levels.
Modern meters have a memory that can recall every test value and the time, and this can be downloaded to your PC.
There are many programs with excellent RN's trained as advisors to help you set up a testing schedule. Being involved with such a "helper" is very worthwhile.
I looked at your Diabetes Tracker Stats, saw your UK input in mmol/l instead of mg/dl (USA). Your daily average level of 10 mmo/l is dangerously high and maximum of 14 mmol/l is suicidal. Sadly to say your blood glucose levels are out of control.
Testing once a day, in your case, will not provide you with adequate data to learn what is causing your blood glucose to elevate. This is the testing regimen first time users must perform before they step down to once a day or even once a week testing. The latter takes months and in some case years to get to. Until you make lifestyle changes - proper nutrition, maintaining normal weight, perform daily physical exercise - you will continue testing 6-7 times a day, or more.
1. Prandial (fasting) - When you wake in the morning before breakfast. No colored liquids, water is OK.
Compare with b4 bedtime reading.
2. Before a meal - To get a baseline measurement. Compare this reading against your post-prandial result.
3. Post-prandial - 2 to 3 hours after a meal when blood glucose from foods you consumed are at the highest level in your body.
Compare with "before a meal" to see if the foods you just ate raise your blood glucose to an unacceptable level (see below for levels). Avoid and stop eating foods that do.
4. Before bedtime - Compare with morning prandial result to see if your liver is producing excess blood glucose while you sleep.
Prandial - 4.0 to 5.0 mmol/l
Post-prandial - <10.0 mmol/l, better <6.1 mmol/l
where < is less than.