It totally depends on the city - in Paris, I've found that's enough time to take the RER transit into the city, have a ~2 hour lunch or walk, and leave with ple

It totally depends on the city - in Paris, I've found that's enough time to take the RER transit into the city, have a ~2 hour lunch or walk, and leave with plenty of time to get back. But of course you could also just search for longer layovers - for international flights, you can take up to 24 hours between flights and still travel on the same fare.