When you book an itinerary with a connection (as a single ticket, not a self-transfer, of course), the booking system will check that it matches the published minimum connection time (MCT) for that combination.
While airports usually have general rules for each of the following standard cases:
- domestic-to-domestic (DD)
- domestic-to-international (DI)
- international-to-domestic (ID) and
- international-to-international (II)
they can also have all sorts of exceptions. All the details are explained in this document from IATA.
Exceptions can be based on:
- The airline(s)
- The specific flight(s) (individually or as ranges)
- The countries of origin or destination
- The terminal(s) involved
- The date
- ...
The full list for a single airport can be several hundred lines long. You can explore that using Experflyer.com (subscription required, but there's a 7-day free trial which does not even need a credit card).
You'll notice that in some cases the list is so long that the underlying GDS cannot even return all of it unless you filter things further.
Here's an example snippet (the first few lines for JFK, out of hundreds):
STANDARD.D/D...D/I...I/D...I/I.
ONLINE 1.00 1.15 1.45 2.00
OFFLINE 1.00 1.15 1.45 2.00
** OR * ARE ALL
AA-AA ID 1.15
AC-AC ID GLSUP US CDS ** - ** COUNTRY CA - COUNTRY CA
AC-AC ID GLSUP US CDS N/A - ** COUNTRY CA - COUNTRY CA
AC-AC ID GLSUP US CDS ** - N/A COUNTRY CA - COUNTRY CA
AD-AD ID 1.45 CDS N/A - B6
AF-AF ID 1.00 CDS ** - ** TRM 4 - ** COUNTRY IE - ALL
AF-AF ID 1.25 CDS ** - ** TRM 4 - ** 27APR19 - INF
AF-AF ID 1.45 CDS N/A - ** TRM 1 - ** 27APR19 - INF
AM-AM ID 1.45 CDS ** - ** TRM 1 - 2
AM-AM ID 1.45 ALL - FLT 3000 - 5699 CDS ** - **
AM-AM ID 1.45 CDS N/A - ** TRM 1 - 2
AM-AM ID 1.45 ALL - FLT 3000 - 5699 CDS N/A - **
AM-AM ID 1.45 CDS ** - N/A TRM 1 - 2
AM-AM ID 1.45 TRM 1 - 2
AS-AS ID 1.45
AT-AT ID 2.00 CDS ** - **
AT-AT ID 2.00 CDS N/A - **
AT-AT ID 2.00 CDS ** - N/A
AT-AT ID 2.00
(and it goes on for hundreds of lines...)
This means that the standard MCTs in JFK are:
- 1h for Domestic-Domestic,
- 1h15 for Domestic-International,
- 1h45 for International-Domestic, and
- 2h for International-International.
But then start the exceptions (which can go both ways: some combinations may have shorter MCTs, others will have longer ones).
A flight combination not matching the published MCT will not even be shown in search results on booking engines, much less be bookable.
So if you book an Air China to Delta international-to-domestic connection via JFK, for instance, while the generic ID MCT there is 1 hours 45 minutes, this line:
CA-DL ID 6.00
tells us that they won't sell you a connection below 6 hours.
The rules and many exceptions are built by the airports and airlines based on their experience of the "general" cases and the many exceptions. In this case I suppose they think people arriving on Air China are very likely to spend a lot of time at passport control, so they won't sell them short connections. There may be similar exceptions based on extra security checks.
In other cases the exceptions will be based on combinations of terminals, for instance.
Note that some combinations are marked SUP or GLSUP. Those connections are just not allowed (as a single ticket).
So if you find a single-ticket itinerary with a connection, it means that the airline and airport think it's doable. Are they always right? No. Obviously they can't predict a meltdown at passport control which turns your 1h30 ID MCT into 2 or 3 hours waiting in line. But it's their responsibility to take care of you and rebook you.
It's a balancing act for them: shorter connections are easier to sell, but if they go too short, they have to bear a larger risk (this is especially true for flights covered by EC261 and their equivalents, and was a primary motivation for it: prevent airlines from advertising and selling completely unrealistic connections).
Now if you want to know if a connection is tight (i.e. exactly at MCT or just above, which usually means you need to hurry quite a bit, will not have time for shopping or using the lounge, and the risk of a missed connection is higher) or not, you can indeed look up the MCT for that specific combination on ExpertFlyer and compare the actual connection time. But remember that flight schedules can change, so even if the connection has a lot of margin when you book it, it may be reduced to nothing if either flight changes.
Now to answer your specific question, based on standard MCTs (ignoring exceptions other than a few generic ones like terminal-based -- note that sometimes the terminal-specific rules are inside airline-specific rules which I have ignored):
- LHR: ID and II 1h30 (but 1h45 for T5<->T4)
- CDG: ID and II 1h30 (but up to 2h30 for T1<->T2G)
- JFK: II 2h
- LAX: ID and II 2h
- MEX: ID and II 2h (but 2h45 for T1<->T2)
I've looked up quite a few but haven't found anything beyond that yet. Most of the usual suspects are between 1h and 1h30. So for now the record seems to be 2h45 for a MEX T1<->T2 connection.
At the other end of the spectrum:
- MUC: all standard 45 minutes, but 35 minutes within T1 and 30 within T2
- ZRH: all standard 40 minutes
- VIE: all standard 30 minutes
- ...
Including exceptions, the highest I found until now is the 6h China Airlines -> Delta ID in JFK, but it's a lot more difficult to list those exhaustively, so there could be more longer ones.
