Work backward from your next boarding time
Starting from the next flight's boarding time and subtracting re-entry security, transit back to the airport, and a safety buffer reveals the real window available for anything outside the terminal — usually far shorter than the raw layover length suggests.
Visa and re-entry rules decide the question first
Before transit time even enters the calculation, whether you are permitted to leave the airport at all under your visa and transit status has to be confirmed — this varies enormously by nationality and destination.
A conservative rule of thumb
Many experienced travelers treat anything under about six hours as too tight to leave the airport reliably once transit, re-entry screening, and buffer are all accounted for, reserving outings for genuinely long layovers instead.
Questions people ask
What is the biggest risk of leaving the airport on a layover?
Underestimating return transit time or re-entry security length, which can turn a planned outing into a missed connection with little warning.
Is luggage an obstacle to leaving the airport mid-layover?
It depends on whether short-term storage is available at that specific airport — carrying full luggage on a short outing is rarely practical.