I have to disagree with the 30 days
i have a silver XL molly and the period of time is more like 70 days.
i have heard of longer but not much.
mine is now getting quite heavy and would expect her to drop in about 1-2 weeks and i put her in one of my breeding tanks in mid may.
i think standard black mollies tend to be between 30-40 days but how a balloon molly differs from mine or the standard i'm unsure.