I process a lot of film. Monday is film day and multiple people bring their film to me. I do over a thousand rolls a year. I use XTOL because I can re use it. I mix 10 liters for my developer tank with a floating lid and I run 150 rolls a film through it undiluted. In general I figure +15% every 50 rolls but in reality I do smaller more frequent increments than that. I have been doing this for 15 years. Though I have gone through phases of diluting 1-1 and 1-2. I like straight because the extended developing time gets too long with the diluted developer. I have printed from this all these years as well and I have always been impressed that the 150th roll in the tank prints just as well as the 1st roll. I date and number the rolls I process as well as track the date and number of rolls I have put through a batch of developer so I can see differences and really I can't see any differences.
How long do you feel it lasts in a deep tank? Not quantity of film processed, but length of time in the tank.