How long does just the NASCAR itself last?

It all depends on the length of the track, and if it rains it will shortened. I'll give a top 5 list of some tracks and their number of laps:

Daytona International Speedway-200 laps-500 miles

Bristol Motorspeedway-500 laps-263 miles.

Texas Motor Speedway-334 laps-501 miles.

Indianapolis Motorspeedway-160 laps-400 miles

Talladega Superspeedway-188 laps-500.08 miles

A race can take anywhere between 3 and 4 hrs.