A full vial is a day's dose (16 hours, accounting for normal sleep time), which can be split up.
Brewing Potion of All Potential will take at least a couple of hours.
There are a few days left in the gathering portion. Once you transition from the gathering area to the brewing arena, you basically have one 24 hours period to brew your potion, though you can submit yours at any time prior to that limit being up.