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.