Originally Posted by ITA Hacker
For US/CA/PR/VI trips, anything over a 4 hour stop is considered a stopover, and most fares within US/CA/PR/VI don't allow stopovers. Generally this will cause a break in the fare and an increase in cost. Even when fares allow stopovers, there's often a surcharge applied.
For most international trips, the limit is generally 24 hours. And international fares more often allow stopovers, often without a surcharge.
There is the exception (supposedly) if your layover is between the last flight of the night, and the first flight of the next day. Then the domestic layover can be greater then 4 hours.
Steve B.