I quite often forget to take my 500mg of Metformin, and don't see any difference when I do finger prick tests.
It's got a half life in blood of around 12 hours, so after twelve hour half the dose has gone (Not *all* the tablet ends up in your bloodstream, around 60% or so, I believe.)
So I don't think missing tablets for a couple of days will cause much of an issue.