Hard to quantify precisely, of course. It allowed putting down cities 6 and 7 as fast as I could get the settlers out, instead of the usual small delay while waiting for happiness from religion and CS quests. But cities 6 and 7 ramp up slowly (because they're your worst sites and don't get the Tradition freebies) and don't meaningfully contribute towards the key milestone dates of Guilds and Education. Probably not more than two turns in the end all told.
It would have helped more for other civs than the Dutch who already have a good early game happy ability. For Korea, the FoY would accelerate cities 4-7 instead of 6-7, probably speeding Education by a few turns, and Education speed is even more important for Korea to kick in that specialist bonus. I could see Korea gaining 4-5 turns from a Fountain of Youth map.
It would have helped more for other civs than the Dutch who already have a good early game happy ability. For Korea, the FoY would accelerate cities 4-7 instead of 6-7, probably speeding Education by a few turns, and Education speed is even more important for Korea to kick in that specialist bonus. I could see Korea gaining 4-5 turns from a Fountain of Youth map.