Potassium Oxalate developer does not exhaust, unless it gets contaminated. It should be replenished regularly with fresh developer by topping off the storage bottle as needed. It gets darker with use as it becomes loaded with metal. Periodically filter it through a coffee filter to remove larger particles and any other suspended debris that may have accumulated. You may find greenish crystals in the bottom of the bottle. This is a sign that the solution is saturated. They are not a problem, but if you want to get rid of them pour the developer into another container temporarily and fill the bottle with water. The crystals should dissolve after a while. Then dump it, or maybe see if it still works as a developer.
Ferric Oxalate sensitizer, at least the B&S kind which is what most of us use, has a fairly short shelf life, typically 6 weeks to 6 months in liquid form. As it ages, ferric ions become ferrous which is the most common source of chemical fog (as opposed to light fog) in the Pt/Pd process. Because ferric oxalate is an “ill-defined compound”, it’s behavior can be a little unpredictable. If you are buying it in liquid form, buy 25ml bottles, and only what you need. If you are mixing from powder, unless you are doing a lot of printing, only mix up 25ml at a time. Unless your printing volume is fairly high, my recommendation is to buy it dry pre-measured in 25ml bottles.
As for coating volume, I can give you some rules-of-thumb but don’t be surprised if you need to adjust them a bit for your conditions. The volume of solution needed to coat - let’s say an 8x10 image area (~ 80square inches) - is highly dependent on the absorbency of the paper. In general, papers with a hard smooth surface and heavy sizing (Hahnemuhle Platinum Rag is a good example) require less solution than those with a softer and/or more textured surface, or less sizing. I find most papers require 1.5 - 2ml of solution for this image size. In drops this translates to somewhere between 12 and 16 drops each of ferric oxalate and metal salts. Drops are an imprecise way of measuring which I abandoned a long time ago.
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