Replenished XTOL lasts for years.
Xtol in my experience is very long lasting replenished. Years. My stabilized seasoned replenished numbers are just about +20% in development time, and my replenishment rates are 60ml per 36 exp roll. Remarkably stable.
One really useful advantage of using replenished developer is that your developer is always at room temperature. I just adjust development time as room temperature varies.
I'm currently using HC-110 (dilution E) in a replenishment regime. When my bottle of HC-110 replenisher is gone, I expect I'll continue the experiments on using HC-110 as its own replenisher that are documented here on APUG.
If I had the space and didn't have as much HC-110 as I do, I would most likely be using X-Tol or T-Max RS (which is another good option).
How do I replenish? With a glass of dry wine or a Bombay Gin Martini.
Sex is not a bad way to go either.
marciofs, read Kodak's publications. They are very clear, and also explain the pros and cons related to replenished development. See Kodak's XTOL technical publication for specific instructions on how to run replenished XTOL. For a more general discussion about replenishment, see the section on replenishment in the following publication:
Other than cost savings, film processed in seasoned, replenished developer looks better to me.
I have experience with replenishing two developers: Kodak Xtol and Edwal Developer 12 (commercially available from Photographers' Formulary as simply 'Developer 12').
Both are incredibly nice developers and I have about six years of combined experience with them. The Xtol batch I used was active and stable for the five years I used it. The only thing you have to worry about is that if you don't develop any film for a couple of weeks, replenish the working solutions with about 70-100ml anyway.
After my divorce I went through a period of creativity problems, and didn't shoot/develop any film for about five months. I also didn't replenish the Xtol. It just sat in its container in the basement. Then one fine day I decided to end the spell and run some film. I mixed some fresh Xtol, replenished my 2 liter working solution with 300ml fresh Xtol, and I ran a roll through it, and it came out EXACTLY as the last time I had used it. Is that incredible or what?
Today I shoot way less film than I used to, so I have moved to developing film using a 2-bath method, but some day when I have more time for photography I'll probably do replenished developers again. Super sharp, fine grain negatives that I get better tonality in my prints from than stock or dilute developer.
Just try it, and have some fun. It's an amazingly economical way to process film too!