One thing I must do is to up my 1L working bottle to 1.8L to provide more cushion for process adjustment.
With the 2002 XTOL I mixed a few days ago, I made the working solution a 2L bottle, because one of my tanks (the Yankee Agitank) requires up to 1.9L (64 oz.) for each rack of film (depends on format, but 9x12 at 1.425L and 4x5 at 1.9L are all I'm likely to put in there).
What happens when you don't use the developer for a while depends strongly on your storage situation. If you have very impermeable bottles for both reserve stock and working solution, you can probably store the working solution for months and see no change; if you don't, you'll see oxidation, which weakens the developer, but you won't see an increase in post-development products (like bromides), so replenishing will tend to make the developer more active. This is why there's the advice not to replenish if you won't process enough film. Kodak assumes multiple rolls
per day in their replenishment documents (I saw a video recently, from a year or so ago, where the videographer toured a commercial dip and dunk lab -- they process hundreds of rolls a day, and have the Thesean-same C-41 chemistry still running after ten years), but many people's experience is that you can run replenishment with good consistency if you process a couple rolls a week.
With XTOL, using the developer as its own replenisher (as with Flexicolor or Fuji C-41) and seasoning or using a starter to start the regimen, if your storage is tight enough (low permeability bottles, near-zero airspace), it seems the frequency of use is less important than it would be with, say, D-76 or HC-110, in which the replenisher isn't the same as original working solution (it's more similar, in D-76, to a seasoned stock solution). Grant's experience and the more common may differ mainly in terms of the kind of storage. Glass or PET bottles will let less oxygen into the bottle during storage than polyethylene like Datatainers or accordion bottles.
I recently found my old Diafine, stored in 1L commercial drinking water bottles with air squeezed out. They're still squeezed in after twelve to fourteen years (I don't remember when I last used them), and the Solution A (which contains the developing agents) is not dark or tinted -- I'll test it sometime soon, but it's probably still good. If low permeability storage can keep Diafine (another phenidone-based developer) for more than twelve years, it should allow keeping XTOL for a couple weeks or more without requiring replenishment just for aging. Put another way, if your XTOL isn't oxidizing in storage, replenishment just for time (not for film processed) will do more harm than good, pushing your working solution back toward an unseasoned state.
