I was all out of proofide, so for those that say it lasts them 10 years they probably haven't treated as many saddles as I have in the last 2. I don't use it too heavily or too frequently, but between my saddles, treating saddles for the coop, and lending some to friends mine was gone, and I had a 1970s vintage saddle in excellent condition, but that probably hadn't been treated with anything in over 30 years.
I also had a little tab of beeswax, an almost empty Mink oil tin, and a fully stocked kitchen, so I put the beeswax in the tin, and melted it on the stove, then added olive oil until the resulting mixture was soft enough to spread. The resulting paste is thicker than proofide by quite a bit, but when I rub it onto a cloth I can spread it thinly onto the saddle, or melt it onto a warmed saddle.
Olive oil is widely recommended in the maintenance of horse saddles which use similar hard leather, and beeswax is used in many of the products folks use on their brooks saddles including Proofide. The cost of a large shoe polish tin's worth is less than $2. The results speak for themselves.
70s Brooks Professional by
mattface, on Flickr