One issue with leaving the machine either running, or turned off, is tar deposits on the rollers happening. Using RA4 chemistry and doing a printing session of a couple of hours, I would then go inside for lunch, or at least a break, for around an hour, then return. I tried turning the machine off, and other times I just left the machine running, and a couple of times I turned the machine off for 30 minutes, then switched on for 10 minutes, with another 30+ minute break.
I would then return to finish my printing session, sometimes I would notice tar deposits happening on the paper, which with hand washing and usually running my fingers over the print in a tray of water, the deposits would easily remove.
As the 2.5 litre volume is good for approximately 40 20cm x 250cm prints (8"x10") if I was anywhere near that level of usage, I would discard the developing bath.
If I was going to keep the developer for another session, either the next day or at the most, in three days time, I would always drop the developer with a hot machine into large glass bottles, use glass marbles to eliminate air, then store for re-use.
In general I found that as the developer is not replenished, the tarring of things sort of escalates if one isn't running paper through the machine. I have no idea why, but it is something I understood was just part of the way it is. In another life I worked in a lab where we had a 1.9m wide RA4 paper processor, which was replenished. Tarring of rollers and prints usually didn't happen as the replenishment system seemed to keep things clean.
These days I use my Printo for B&W RC paper, it works wonderfully, same times as RA4 but around 34ºC.
