Well, it took me quite a few months to get pregnant but the folic acid was never a concern (in a kind of overdose way) they were always just keen to make sure that I was taking it (my levels were good and was off the pill, just not getting pregnant). They were going to let things go naturally for a year before seeing if there were other problems, so that would have been 12 months of folic acid before conceiving...
I seem to remember reading it is good to be on in at least 3 months before conception and as no one can really be sure when they get pregnant most probably end up taking it several months extra.
Xx