The fantasy players' degree of being a conversion audience is probably overrated, IMHO. However, if they are targeted, it's senseless to adopt anything year long. Annually, the Sporting News (I think one of the first to hit newstands every year) and others publish their fantasy issue, e.g. one list:
Fantasy Baseball Magazines
Even the casual gamers like myself or the casual fantasy player usually pick up between 2-4 copies of various issues for comparisons and info. So, a small ad in 2-3 or even 1 larger ad in one of the more premier sellers (in numbers) would probably net 100X the investment of any one regularly appearing ad in any one issue. Preseason is the time to roll out marketing and opening day is the time to have the game ready for those who may potentially come aboard.