I thought that I would add a note to this old thread.
I have been using Cella for many years. I only use it six months out of the year as my fall/winter soap. I use something mentholated in the spring/summer, usually LEA, sometimes artisanal soaps. I only shave three times per week so a tub of Cella usually lasts me two years. I have never had a tub go bad. I do bloom the soap before use while my brush soaks so it is not like I keep it exceptionally dry. OTOH, I have never lived in a hot climate and that may have something to do with the problem some people have. Keep it out of the Sun, under hot lights, etc.-anything that would cook the soap. Also, don't contaminate the soap with bacteria from your unwashed hands, etc.
Sometimes near the end of the two years when I am at the bottom of the tub, the fat congeals into hard small nodules but that does not seem to effect the shave. I have seen that happen with other tallow based soaps too. I think when some people say it goes bad and starts to stink, it is just the scent having dissipated to the point where you can now smell the beef tallow, which is offensive to some people.
Keep in mind that Cella was originally designed as a barber's soap and I think that they assumed that the barbers would tear through it pretty quickly. I don't think it was designed to last for multiple years on a shelf. So, if you are going to use Cella as one soap in a rotation of a dozen, it may not last for years and years. But, if you use it regularly, you should be able to get through a tub without any problem .
I keep my butter out at home, i.e. not in the refrigerator and I have never had my butter go bad either, but I am usually using it up within a month or so before I take a new stick out of the refrigerator.
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