It should go to F8 next, if anyone has the cave location to f8 i could check
My theory is the fountain has a set number of people that can grab it. here why
hour 1 - it changed really quick like within 50mins
that evening - it started to changed every 1.5hrs (because less and less people needed it)
this morning to now - its around 5-6hrs between change (once again, because less and less people need it)