I’ll put it quite simply, if you’re making an international trip and have any connections, try to plan at least a 2 hour transfer in each connecting airport. This is especially important once you start transferring through other countries during your trip.
In my own experience, two hours is about the minimum you want. Three hours is preferable, but not always possible given the nature of international flights. In most cases, your airline will only have one or at most two flights each day between where you are and where you’re going. They usually run one inbound and one outbound flight at roughly the same time each day from international airports.
The two hour minimum, in most airports gives you enough time to pass back through airport security, find your gate, change to a different terminal, stretch your legs, visit the bathroom and get some decent food. Also, airports are like shopping malls with million dollar buses departing every few minutes, so there’s shopping as well. But the biggest reason is that you don’t want to miss a tight connection because you were stuck at the only security checkpoint when 10 international arrival flights disembark at the same time.
Using my recent flight to Taiwan as an example, I made the mistake of accepting flights from my travel agent with 90 minute layovers (on schedule). Due to delays, however, those 90 minute transfers were really more like 50 minute mad dashes.
In DC, I had exactly 40 minutes from the time I got off my first flight until the scheduled departure of my flight to Tokyo. My inbound flight parked at the ass end of Concourse D at IAD, my outbound departed at the ass end of Concourse C. This may not sound so bad, until you realize that C and D are in the same building. Worse when you realize that the gates are numbered like this:
C1 -> C30:D1 -> D32
That building is roughly a mile long. So I had roughly 30 minutes to get off the plane, walk through the crowd for a mile, and get right back on a plane. No time to even take a squirt. Not fun.
In Tokyo it was more of the same. We landed 30 minutes late, and I managed to get to my gate and check into my final flight about 5 minutes before time to start boarding. As luck would have it, that flight was delayed by about 15 minutes, so I actually DID have a few moments to grab a snack.
So that’s my rambling about transfers. If you have the option, make your transfer at least 2 hours, preferably 3 for international flights/transfers. Domestics are a bit easier but can still be a hassle if you have less than an hour between flights.