MattKing has explained sheet film development with replenished developer pretty neatly but because I've used the same replenished Xtol batch for eight years I'll add some extra thoughts.
I replenish at the rate of 90ml per film (35mm 36exp = 120 roll = four 4x5 = one 8x10) and the developer activity has stayed the same for years. Typically I develop four 4x5 sheets held in a slosher with 1 litre of developer in a tray. Excuse the curious line of thought but each 4x5 sheet lying on its back in the tray has 250ml of developer above it which contains enough developing agent to do about 11 sheets. So after one cycle with the slosher the developer activity in the tray has dropped about 9%. This decrease may not be discernable to the eye but might be discoverable with careful densitometer tests. In any case replenishment returns things to the starting condition and an original batch of developer can be refreshed essentially forever.
A more subtle consideration is that replenished developer activity is not solely limited by exhaustion of developing agent but also by the accumulation of development by-products. These include some dyes (antihalation, acutance, speed trimming, etc) and some free halide, typically bromide, a restrainer. These by-products can enhance the developer's ability to deliver improved image quality compared to one shot developers. Again, the by-product improvement of development outcomes is held consistent by the replenishment routine.
I replenish at the rate of 90ml per film (35mm 36exp = 120 roll = four 4x5 = one 8x10) and the developer activity has stayed the same for years. Typically I develop four 4x5 sheets held in a slosher with 1 litre of developer in a tray. Excuse the curious line of thought but each 4x5 sheet lying on its back in the tray has 250ml of developer above it which contains enough developing agent to do about 11 sheets. So after one cycle with the slosher the developer activity in the tray has dropped about 9%. This decrease may not be discernable to the eye but might be discoverable with careful densitometer tests. In any case replenishment returns things to the starting condition and an original batch of developer can be refreshed essentially forever.
A more subtle consideration is that replenished developer activity is not solely limited by exhaustion of developing agent but also by the accumulation of development by-products. These include some dyes (antihalation, acutance, speed trimming, etc) and some free halide, typically bromide, a restrainer. These by-products can enhance the developer's ability to deliver improved image quality compared to one shot developers. Again, the by-product improvement of development outcomes is held consistent by the replenishment routine.


Even with a heating pad under the tray I find my temps go down pretty quickly in the winter and the only way to really control is to use dbi. And it controls for other issues as well of course. If you are developing sheet film I think it's the way to go. And with infrared googles it's supposedly even easier, though I've never tried that. Of course, two sheets at a time takes longer, but at $5 a piece of film why hurry?