Inspired by this Worldbuilding question about making a booby-trap with a very long shelf life. The idea is to suspend one lump of fissile material say $3/4$ of a critical mass above another of the same mass. When one is dropped on the other there is some sort of violent reaction.
Let's not worry about how effective the trap would be. I am principally interested in how quickly it decays over time. Let's say the elements are made of $U$-$235$ whose half-life is about $700$ million years. Naievely you would expect the two elements to last about $500$ million years before the total mass becomes subcritical.
But I understand it's more complicated than that because the half-life formula doesn't account for the interaction between the atoms. Obviously when a uranium atom decay it shoots out particles which make nearby atoms more likely to decay. So a $2$ kg element should decay to $1$ kg faster than a $1$ kg element decays to $1/2$ kg. And an element of critical mass will decay suddenly and explosively.
So how long should we expect the booby-trap to remain viable? Of course there are loads of variables I have not considered or specified like the density or shape of the elements or how reflective the surroundings are. Feel free to set those variables to whatever is most convenient to getting a broad estimate. Are we talking one year or are we talking a hundred million years?