Just over 18 hours to get a full bar here. It really depends on your level whether trading 1 FG for a full bar of stamina is worth it. But that full stamina easily translates into 3 lvl 100 Manyus and a bunch of lower level manyus when used on the daily map, at least for me. And that's a way better deal than the 1 Manyu for 3 FG that the shop has on offer. I'm actually tempted now to use an FG to do some additional card farming on the event map! 80 stamina map = 1.6 Manyu, while the SG is worth 3 manyus. So spending one FG on stamina currently yields at least 6.4 manyus for me. But, you need a high level (and thus have high maximum stamina) to do that.
Once you get at a high level, and you're mostly settled on your unit setup, the value of FGs shifts quite a bit. You might want to save up for some waifu you really want, but otherwise, it's really a matter of X amount of Manyus vs a fifth of the cost of yet another bronze pull.