Depends. A terrorist weapon is likely to be detonated at ground level, which is going to make it real dirty - throwing lots of debris in the air. The Bigger the bomb, the more debris. Know the prevailing winds in your area. If you are reletively close (25-50 miles) to the blast site, move laterally out of the wind channel the fallout will track on and then get up wind.
The duration of fallout danger depends on how high into the atmosphere the debris kicked up by the bomb is injected as well as weather conditions. If something like this happens, those familiar with the weather channel are going to have a leg up avoiding it.
Depending on the tonnage of the bomb and where and how it is detonated, Fallout danger could range from a week to months, and could be anything from a local, to national or international problem.