How long can i take otc prenatal vitamins for?

me and my husband are planning a baby, but to be honest alot of things can change over this year..so we dont even have a date or month when we want to officially conceive lol but i want to be prepared so if i take otc prenatal vitamins now will it be safe even if i get pregnant a year from now?? btw i was thinking of taking over the counter pills for now., later on the road maybe prescription ones..whats the best ones? i heard gnc?