Airports publish a minimum connection time, and airlines won't sell a connection on one ticket shorter than that. Legal isn't the same as comfortable.
Rules of thumb
- Same terminal, domestic, one ticket: 45–60 minutes is usually fine.
- International, one ticket: 90 minutes to 2 hours, more if you must clear immigration or security again.
- Arriving in the U.S. from abroad: you clear immigration and customs and re-check bags at the first U.S. airport. Allow 2–3 hours.
- Separate tickets: 3+ hours (see self-transfer).
On one ticket, a missed connection is the airline's problem
If the first flight was late, the airline must rebook you. That's a strong reason to keep connections on one booking.
Turn long layovers into mini-trips
With 6+ hours and the right visa, you can leave the airport. Store your bags, take the train into town and return with time to spare. Some airlines even offer free stopover programs.