I haven't had it long enough to know, but I expect it to be quite long. If it is true that the role of the ascorbate at this pH is to replenish the Metol and not superadditivity, the ratio of remaining ascorbate to Metol will not be important. The pH and bromide content of used or aerated developer is more important. There wont be any bromide build up due to aeration, and most will use it one-shot so bromide won't be a problem. The rather generous quantity of borax wont allow much change in pH during one shot use. I'm planning to leave a pint or so in an open container to see if it's still good after a month or so.