No it won't wear off, it builds up in your body over a few weeks and then you just keep taking it to keep it stable, that's why you shouldn't just stop taking them without tapering.
I'm not taking any vitamins at the moment, all my levels seem ok (except maybe iron, getting that rechecked soon) . I was taking a multivitamin with everything in it twice a day when I was first diagnosed. You can get them in supermarkets. I had them on prescription though because I can't have any with calcium in