Much of the research suggests that phenidone is not very stable in solution. However, exactly what does that mean? In water? In alcohol? In acetone? In diet cola?
I have found that phenidone is very stable in the Part A Stock Soluition of Pyrocat-HD. And I am using Phenidone A, not one of the more exotic variants of Phenidone. My Stock A solutions are good for up to six months, or slightly more. There is absolutely nothing in my experience to suggest that one will get better results by mixing the Phenidone from scratch for every working solution.
The key to the stability of Stock Solution A, at least from my perspective, is the balance of preservative, i.e. sodiuim bisulfite, in Stock Solution A. There must be enough to preserve the reducing elements, but not so much that all staining is eliminated. I think we have just about the right amount in the Pyrocat-HD formula, though for sure with certain films and long development times one may need to add a pinch or two of sodium sulfite to reduce the possibility of general fog from developer oxidation.