Uranium-$235$ has a half-life of about $700$ million years and a critical mass of about $52\,$kg. That means if you take a large number of $U$-$235$ atoms, say a $26 \,$kg amount, and put each atom in a separate sealed container, then after $700$ million years about half of the atoms will have decayed.
Now suppose instead we have just a $26 \,$kg lump of the stuff. In this case the atoms are allowed interact with each other. And by definition of fissile material they do very strongly: Whenever one atom decays it shoots off particles with hit the other atoms and make them more likely to decay.
If we had a critical mass this effect would compound itself and we'd get a nuclear explosion: There would be a sudden explosive decay that would continue until we get below critical mass.
Since we only have half a critical mass the decay rate will not be so fast or sudden. But we'd still expect the interaction make half the atoms decay in less than $700$ million years.
How does one compute this half life (edit: time for half the atoms to decay)? Feel free to make whatever assumptions about the lump of material as desired. For example an idealized shape/density or some external force pushing the atoms together to prevent the lump blowing itself apart.