I have been processing C41 and E6 for many years, and am still using Nikkor tanks and reels in a water bath. To determine developer life and activity, I suggest that you shoot a roll of the MacBeth color chart using your normal film and camera. Don't bracket exposures. Shoot a set in full sunlight at the proper exposure. Fish out the leader of the roll and store it in the fridge in a capped film can. Use this for your test sequences. Cut off and develop a few frames to determine your developer's activity.
In this regard, a calibrated film scanner can be of benefit to determine if you are getting results consistent with fresh chemistry. It's certainly not as good as densitometry, but gives you a guideline. Of course visual inspection and comparison with your "standard" MacBeth negative does the same. You should count on around two weeks for tightly sealed, full bottles of partially used chemistry. Note that the lifetime for the blix and stabilizer is far greater, typically a month or more. It's the developer that oxidizes and fails first.
M. T. Sandford