Yes, you are generally allowed to leave the airport terminal during a layover, provided you have the necessary travel documents to enter the country. For domestic flights within your own country, you can exit and return freely, though you must pass back through security screening, which can take 30 to 60 minutes. For international layovers, you must pass through immigration and customs, which may require a valid visa or a "transit visa" depending on your nationality and the local laws. Experts typically recommend a layover of at least 6 to 8 hours before considering a trip into the city, as you must factor in the time to deplane, clear immigration, travel to the city center, and return at least 2 to 3 hours before your next flight. Some airports, like Singapore Changi or Amsterdam Schiphol, are famous for being so close to the city that a quick excursion is very easy. However, if your layover is short, the risk of missing your connection due to traffic or long security lines often outweighs the benefit of a few hours of sightseeing.