Contrary to the movies, they don't glow all that long. Off the top of my head, you can apply the 7/10's rule. For every 7 periods of time, the residual radiation decreases by a factor of 10. Some neutron induced activity in the vicinity of ground zero will last longer, but still not that much of an issue. One would use an airburst, anyway. Go for the static overpressure blast effects, and better thermal. Wouldn't be that much residual to start out with.