A tornado watch is issued by the National Weather Service when conditions in the atmosphere are favorable for tornadoes to develop.
According to the National Weather Service, tornado watches are typically issued to span approximately 4 to 8 hours, though it is dependent on the weather situation at hand.
During a tornado watch people should remain on alert for the possibly of severe weather in the form of large hail, damaging wind and tornado development. Storms can and often do form very quickly and many times with very little advance notice.
For comparison's sake, a tornado warning is different in that it means an actual tornado has been observed to have touched down, or is indicated by radar.
During a tornado warning, people should seek immediate shelter in the lowest level of building or to a small interior room away from windows.