Actual keeping properties of Ethol LPD I don't know about, but I've had it last for 6+ months, (when I had no darkroom access), at 2+1 dilution in a partially filled brown amber glass bottle. I doubt it would last two years even as a concentrate.
Ethol themselves claim that LPD lasts for six months. I normally use it up before six months passes by so I have no way of verifying working solution or stock solution, only 2:1 mix with water.