I am not sure about the oxidation aspect, but every developer has a certain minimal stock required to develop 80sq inches of film. D-76, Xtol ID 11 it is 100-150ml. With Rodinal I have seen varying amounts from using 10ml of stock all the way down to 3ml.

You could expose some sheets on an 18% grey card, number the exposures and then process with your method. After completion, check the densities by printing some contacts and see if there is any visual differences. If you have access to a densitiometer you wouldn't have to print to get results. This should give some idea at what point the devloper starts to go bad. I know that it may sound like a waste of film, but it is probably the only way to know the answer.

Keep track of time and temp film is in developer, and how long the developer is out and how long each sheet is developed for. Since your hands are busy, a small tape recorder is handy for making notations during the process.