Tornadoes can last seconds, or they can last hours. Some of the weakest versions have their path measured in yards; some of the longest can go over 100 miles.
But when a tornado is forecast to stay on the ground considerably longer than a typical twister, meteorologists will refer to them as "long-track" tornadoes.
Most tornadoes last less than 10 minutes, according to NOAA's Storm Prediction Center. And the average path length is about 3 1/2 miles. There is no hard definition to be considered a "long-track", but think 15-20 miles or more.
It takes a potent supercell with extremely favorable atmospheric conditions for a tornado to survive for an extended period of time and thus longer-track tornadoes tend to be on the stronger side. A forecast including the potential for long-track tornadoes should really be taken seriously.
The record for the longest-track tornado is the "Tri-State Tornado" in March, 1925, on the ground continuously for 219 miles. The deadly tornado outbreak that just swept through Kentucky in December was on the ground for nearly 166 miles.