I, too, tone in batches long after my prints have been washed and dried. I've never noticed that elapsed time made any difference. I've successfully toned prints I made years ago . I soak the prints, immerse them one at a time in the toner (1 to 10 or so for MGIVFB) for three minutes, and put them in the washer (Versalab) for an hour. I keep an untoned print in a tray of water next to the one I'm toning to be certain I've gotten the effect I am looking for, which, with Ilford, is mainly a deepening of the blacks and, as a consequence, a very slight boost in contrast. Unless there's a way for ME to soak in a bath which will prevent deterioration and preserve me while I'm still alive, I really don't give a rat's rump about the archival permanence factor of the prints.