I bought a kilo of phenidone from Ilford in the early 1970's and the remainder is stilll fully active. I broke it up into 10 100 gram lots, bottled it and sold 7 bottles at the time to friends. I have kept the remainder cool, dry and stored in sealed amber glass containers.
Dry phenidone powder has a very long shelf life - many years. In water based solutions, the shelf life is limited. Water based solutions of phenidone can lose strength and fail without warning.
On the other hand, phenidone dissolved in glycerine, glycol or triethanolamine has a very long shelf life (as long as no water is added).