It seems to last pretty long if you squeeze the bottle to keep out the air as you use it. Maybe 6 months at least. I usually mix it straight from the bottle, but they recommend that if you want to keep it for a long time, to mix a stock solution and dilute the stock to the working dilution. I used Neutol WA with a few different papers and found it worked best at 1+7.
I have some of unknown vintage, got it for a couple of bucks at a swap meet! It has visibly darkened (new stuff looks pretty clear) in the unopened bottles but works fine.