What time of day should I take my metformin, morning or night?

"For oral dosage form (extended-release tablets):
Adults—
Metformin alone: At first, 1000 milligrams (mg) once a day taken with the evening meal. Your doctor may increase your dose if needed until your blood sugar is controlled. However, the dose is usually not more than 2500 mg per day."

Source: Mayo Clinic

Take metformin with a meal, unless your doctor tells you otherwise. Some forms of metformin are taken only once daily with the evening meal. Follow your doctor's instructions.

https://www.drugs.com/metformin.html

Masso is correct like 99% of the time. I just want to add my 2cents as I'm one of those people that can't tolerate this medicine, even at 500mg. Bad diarrhea kicked-in after about a week. I mean really bad! So just be aware that you might be one of the people like me and lot's others. I would definitely not take 500mg twice a day or 1000mg before your body gets used to it and is not causing this side effect.
Check with your health care professional before doing anything other than he/she has prescribed to you.