Nemjo,
Rodinal is a one-use-only developer, to be diluted right before using it, it expires pretty fast once diluted with water (you should not even mix it too long before use, since it will already go bad if only sitting there) - quite paradox, if you consider that as a concentrate it keeps practically forever...
The only developers I'd use multiple times are 2-baths, with divided stages of developing and activationg substances (like Diafine).
Theoretically, you could reuse some developers like ID11, by adding a time factor for every film it is used with - but this is very bad practice, as there is no consistency, and all kinds of stuff from the films keep adding up and interacting with the agents in the developer.
So:
Rodinal: 1x use only!
ID11 and similar soups: use diluted 1+1 or 1+2 for economy - but only 1x.
Or use a 2-bath dev. which can be re-used for multiple films!
As for keeping: Rodinal concentrate keeps forever, even if opened, it just gets darker (like black coffee); other developers should be used up in a few months, after they have been mixed (for powders, like ID11; liquid concentrates should be used up even faster); most will turn slightly yellowish-brown once they start going bad (with the exception of ascorbate-based ones, like XTOL, Ilfosol-S or FX-50, which can go bad without any color changes from one film to the next - this is called 'sudden death').
Fixer: 1l of mixed solution can be re-used for about 6 films (35mm - 36 image, or 120 roll-film); the exact number depends on the concentration of active ingredients, usually there is some advice on the bottle; unopened fixer concentrate will keep for a couple of years, but once opened I'd use it up over a few months - if it gets cloudy/milky, with flaky stuff in there, and starts smelling really bad, don't use it any more.
There are test strips for testing whether your mixed fixer is still OK, but those are expensive and not very exact - you might adopt the 2-bath fixing-scheme used by most German fine printers: use your fixer as first bath for 6 films for half of the fixing time, and a second batch of fixer for the rest of the time; after 6 films, put away the first bath, use the second one as your new first bath, and mix up a fresh batch of fixer for a second bath. This makes extra-secure your films get fixed correctly!
Films: B&W films can be used a couple of years beyond their expiry date, if they have been stored cool & dry (best: in the freezer); if stored warm, they don't keep so long; also, high-sensitivity film can become fogged, and lose some of its speed once it is some time beyond its expiration date.
Roman
