It seems to have different effects on different people. Some might say that it doesn't have any effect until you take 2gms a day (probably 2 x 500mg, twice a day). In the early days after my diagnosis that was very effective in controlling my BG but I think I can safely say that following the old carb advice 20 years ago meant that I eventually had to have other meds.
For me the worst was Rosiglitazone, no longer available in the UK, because it badly affects liver function. As mentioned in another thread, Metformin has an accumulative affect and you may notice a deterioration of BG over a period of time if you stop all together. Perhaps lowering your carbs has had the desired effect and so long as you keep to low carb you might safely be able to forget about Metformin. There's no hard and fast rule, you have to try it and see. Our personal experiences are all very different.