I haven't yet received my chemicals that will let me deliberately put Cu and Fe contaminants into the developer, but I did some tests to see how it currently holds up over 24-hours. I mixed the baseline developer in distilled water, with and without salicylic acid, and did leader tests when mixed, and after 24 hours. Here are the leader densities:
No salicylic, when mixed: 3.62
No salicylic, after 24 hrs: 3.65 (no loss)
With salicylic, when mixed: 3.67
With salicylic, after 24 hrs: 3.57 (small loss)
So salicylic acid made longevity worse? Huh?
Anyway, this shows that my chemicals (especially sulfite) are low enough in Cu and Fe that there's little or no perceptible loss of quality after one day. Maybe I should try this with our hard tap-water.