Personally, it's been between 10-15. However, if I can count the experience of some of my hide tanning peers, that goes into the decades. Salt continues to draw moisture and well if you wash the salt out very thoroughly the alum can also be washed out, lol. It is a very old argument even on the taxidermy net site..think this is why new ingredients are always being introduced, but, how many of those are actually in fact alum based....
..I don't mean to cause argument as I have in fact 'tanned' quite a few skins myself using aluminum sulphate and salt and even sulfuric acid, and salt... and they came out really beautiful. I have 'heard' about disintegrating skins from some very reliable sources. It could be that when water is reintroduced to those extended shelf life skins.. Personally? I might not be around then and if I do live that long? :-) Actually, I'm suffering now from some of the tanning I've done.....but it will not stop ME from tanning them, so really there are MANY ways to skin A Cat and tan them. Oh yes and rabbits too :-). Definately a thread topic here that I should have declined any comments on, lol. Must be my conservative side coming forth! Just a differing thought and one that just might bear some forethought?