No one has figured that out yet. Once we can determine that, we can design containment systems or ways to manipulate the radioactive properties of matter to make new atoms or to eleminate radioactive waste.

It is a function of (a) how unstable the parent element is, and (b) whether it follows a gamma, alpha or beta decay path. Some unstable elements only have to throw off one particle to become stable, a one-step process. Some unstable elements have to throw off several particles in order to become fully stable, which will happen one particle at a time. Different unstable elements have to throw off different kinds of particles in order to achieve stability. What they have to get rid of depends on the initial configuration of the atom. Alpha, beta and gamma decay refers to the types of particle which can be thrown off. Some elements have only one decay path, throwing off one kind or particle; some elements have several decay paths, throwing off more than one kind of particle. Again, this depends on the configuration of the atom that you start with. Alpha decay is the release of positively-charged particles, and beta decay is the release of negatively-charged particles (electrons). Gamma decay is simply the release of excess electromagnetic energy from the nucleus, leaving the mass of the atom intact. Each type of decay proceeds at a consistent rate, but those rates are different from each other. If you know the starting configuration of your unstable element, and you know the rules governing what states are stable and what states are unstable, then you can calculate the type of decay that the element will need to undergo to achieve stability, and if you know the rates that each type of decay will proceed, then you can calculate an element's half-life. You can find out more about radioactive decay at http://hyperphysics.phy-astr.gsu.edu/hbase/nuclear/rdpath.html and http://www.sasked.gov.sk.ca/docs/physics/u8b3phy.htm if you want to.