I suggest a week. If it's going to work (doesn't always) you will have peak effect by then. No point continuing it if it isn't having a satisfactory effect. Some have reported insulins going down over a longer time frame but it's more likely that was from other changes made, not the metformin.
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Eleanor in PA

www.drkellon.com 
EC Owner 2001

Hi Kelly,
Here’s a link to a write up addressing the most frequently asked questions about metformin.  It suggests retesting 1 week after starting metformin.
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Hi all,
What is the suggested timeframe after starting metformin to have a repeat insulin run?
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Kelly Gilmartin/Buddy
Western Massachusetts 
member since 2019
Case History: https://ecir.groups.io/g/CaseHistory/files/Kelly%20and%20Apostrophe%20GCH

Photo Album: https://ecir.groups.io/g/CaseHistory/photosearch?q=Apostrophe