By John Dodge

CHICAGO (CBS) -- Residents in much of Chicagoland typically ride out a tornado warning for anywhere from 45 to 60 minutes, according to data from the National Weather Service.

That's the average time for a tornado warning for Cook County and Lake County in Northwest Indiana.

The NWS examined data for the past 20 years, from 1994-2013.

The average tornado warning time was lower in Lake County, Ill. (15-20 minutes) and DuPage (20-25 minutes).

Farther to the south in Will County, the average time rose to 75-90 minutes.

Tornado warning times were typically lower in counties to the west and north of the Chicago area.

Counties to the far southwest of Chicago were much higher.

Three counties studied--Livingston, LaSalle and Iroquois--had the highest warning times, ranging from 90 minutes to two hours on average.

Farther to the east in Indiana, times dropped to about 30 minutes.

The area-wide average was just under one hour, NWS found.