How do you know when to stop reusing a developer?

If you have x parts per litre of metol in Bath A, when you carry over any amount to Bath B, you still have x parts per litre of metol in Bath A. Carry over doesn't deplete Bath A. Development in Bath A does - but absorption of byproducts probably inhibits that by the second roll.

The metol dies in bath B. Whatever amount of sulfite comes with it may prevent it from immediately dying, but it will surely die fairly quickly thereafter. The amount of sulfite amounts to a very small concentration in Bath B after a roll is developed.

pbromaghin said:

Oh, you will.

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A very good point, all mistakes need to happen to transition from 'I've read the rules' to 'I've had may ass bitten for the first time and now I understand the rules'.

But the time to stop reusing developer is never to start reusing it. The last time I used it was forty years ago in newspaper darkrooms in deep tanks that were continually replenished due to the high traffic But newspaper darkrooms were never geared towards quality when the deadline was always 'now', it was a way to get any sort of image onto the page quickly. So unless you have a vast turnover reusing developer takes you an un-necessary step away from being completely in control by using 'one-shot' developers, and the variety they offer.