Start with the ticket
Is it one booking or two? On a single ticket, a protected connection, the airline must rebook you if the first leg runs late. On two separate tickets, a self-transfer, you are on your own: the second airline owes you nothing. Leave at least four hours for a self-transfer, expect to collect and recheck luggage, and ask whether your bags are tagged through to your final destination.
Know your airport
Terminal changes eat time. At Heathrow, moving between terminals means a transfer route and security again. Frankfurt and Istanbul are huge, so the walk to the gate can take longer than the flight to the next city. Schiphol is compact and Changi links its terminals by Skytrain. Check the airport map before you land, not after.
The fifth gadget: a tracker tag
A Bluetooth tracker such as an Apple AirTag, Samsung Galaxy SmartTag or Tile goes inside the bag and shows whether it made the connection. It is not a guarantee, only data, and the coverage depends on the brand and the phones near your bag. Most airlines allow trackers in checked luggage because the batteries are tiny coin cells, but check your carrier's policy. It is the cheapest hour you will buy on this list.
Leave or stay
Leaving the airport takes more than a taxi. You may need to clear immigration, possibly hold a visa, and return through security. As a rule of thumb, do not attempt it under six to eight hours, and verify entry requirements on the official government site first.
Some hubs make it easy. Changi has offered a Free Singapore Tour for transit passengers, and Turkish Airlines has run the Touristanbul tour for eligible passengers at Istanbul. Eligibility and availability change, so check before you plan around them.
Or stay and wander
Stay airside and many airports still reward the curious. Changi's Jewel has the Rain Vortex waterfall. Hamad International in Doha has an indoor garden. Schiphol has a small Rijksmuseum exhibit. Check opening hours and whether you need to pass back through immigration to reach them.
Build the time budget
Subtract ninety minutes from your wait for boarding and security. Subtract two more for eating and washing. What is left is yours: split it among charging, sleeping and moving. The strip above gives you three skeleton plans. Change the numbers to suit your own flight and your own patience.