This info is probably here somewhere but I couldn't find it when I needed to know.
I emailed Dharma to ask how long the dye would last once mixed and this was their reply:
Once Fiber Reactive Procion type dyes (powdered ones) are mixed with water for tie dye, they are great for a couple of days, and good for a week, and ok for a couple of weeks. Each color is different as to how well it does. What happens over time is that they react with the water, so they gradually become unavailable to the fabric. Old mixed up dyes will look great going on, but more and more will wash out, leaving you with lighter duller colors when the mix gets too old. Some say refrigeration, and adding a little salt ( 1tsp/8oz dye mixture) helps the dyes to last longer, but we haven't done any tests to see how much longer. Professional dyers use freshly mixed dyes.