A properly cleared and fixed Kalitype should be quite stable. Some practitioners, particularly the early ones, were not very careful about these steps, and the results could be very unstable. Still, the silver image in Kalitypes is more delicate that that in silver-gelatin prints, and it is probably more subject to attack by things in the atmosphere and in the paper. Careful choice of paper for archival stability will certainly help. Gold or platinum toning is definitely recommended (selenium may work, too). Treatment of the print with various varnishes that are available now is a possibility, but it really hasn't been tested.