I make my own X-tol and the syrup seems to keep indefinitely. When I want to use it I can dilute 1:20 to get standard stock activity or dilute more for longer dev times. It is always one shot after the dilution, so I don't worry about storing it. It costs just over $3 to make 50mL of the concentrate (0.3 cents per mL of stock dilution). Commercial X-tol comes to about 20 cents per mL of stock dilution, so the economy is in my happy range and it doesn't die unexpectedly. The only tricky part is weighing out sub-gram amounts of phenidone, but that can be worked around.
Stock solution in propylene glycol / triethanolamine:
Start with 13.4 ml hot triethanolamine
25 ml hot propylene glycol
11.5 g ascorbic acid
0.15 g phenidone
Stir until all solids have dissolved (it will take quite a while, and may require the addition of 1-2 ml of water). Dilute with hot propylene glycol to a final volume of 50 ml.
Working solution: The stock solution should keep indefinitely. For use, dilute 1+19 with water containing 60 g/L sodium sulfite — the resulting solution matches the activity of stock XTOL. To create something like XTOL 1+1, dilute 1+39 with water containing 30 g/L sodium sulfite.
