My experience with Tetenal liquid is that it exhausts about when it says it will. Convert that to square inches of emulsion if you process differing formats. Extending dev life by extending dev time has proven unpredictable for me and, since most of my shots cannot be revisited, I throw out the processing chemicals, dev and blix, after very conservative use. The 'stabilizer' seems to last much longer. If residues form in the chemicals, you can filter with pre-rinsed coffee filters once, maybe twice. Chemicals are cheaper than disappointment.