They don't age signifigantly, ever.

I'd say in an average FF, the events happen in about 3 weeks. I feel like the time it takes to travel between places in FFs are relatively short or almost non-existant most of the time. Take FF8 for instance. Walking to balamb from balamb garden should take less than a day, I'd suspect. Every time you complete a big mission and are headed off somewhere else, I get the impression that another dayhas passed. So... after a main dungeon and a boss is about a day. Since theres like 20-30 big events, bosses, dungeons about, I'd say about 3 weeks, but of course, this is all relative, and ahs no real basis at all.