What is the minimum time to consider with a connecting flight in the USA?
Given the extensive time taken for immigration formalities in the US, what is the shortest time to consider between connecting flights in the US, when coming from abroad?
Until now I have always entered the US at my destination. This is not always the cheapest option. I was told that 3 hours is the advised time to go through immigration and boarding of a connecting flight. Online booking sites still offer connecting flights, with a transfer times sometimes even less then 1 hour.
It will make a big difference whether you are a US citizen or not so you should mention this.
I have added a tag non-citizen
There is an official "minimum connecting time" for each airport. For international airports, there are usually separate "minimum connecting times" specified for domestic to domestic, domestic to international, international to domestic, and international to international. At larger airports there may even be longer minimum connecting times when you are changing airlines.
Believe it or not, the data is not readily available online. Here are some places to find an official minimum connecting time, sorted in order from most authoritative to least authoritative:
The official source is the IATA, which publishes the Airline Coding Directory, which costs hundreds of dollars.
A much cheaper way to get the same data is to get the Official Airline Guide. Since minimum connecting times don't change very often, I recommend just buying one copy of an outdated OAG Pocket Flight Guide on ebay for a couple of bucks.
Call any airline involved in your itinerary. Tell them you would like to know the minimum connecting time for the airport in question. They'll know exactly what you're talking about and look it up for you.
Airlines sometimes publish minimum connecting times on their websites or in their timetables, usually only useful when you are connecting on the same airline. But if you are connecting on the same airline, you don't have to worry about this, because they will not book you on a connection which is shorter than the minimum connecting time.
Remember that minimum connecting times are only estimates. Your first flight might be late. US Immigration may be overloaded. The airport train may break down. The security check in for the second flight may be really long.
Due to all the steps involved, international to domestic transfers are always terrible and should be avoided if at all possible. You are literally better off transferring through a third country that has good international->international connections (Vancouver, Amsterdam, and London Heathrow Terminal 5 are all great for international to international) if that allows you to enter the US at your final destination.
So for example if you're travelling from Europe to Detroit, you're WAY better off changing planes in Amsterdam than at JFK. If you're travelling from Seoul to San Diego, you're WAY better off changing in Vancouver than SFO or LAX. Etc.
Like everything, "IT DEPENDS," so please feel free to ask a more specific question -- at the very least we need to know what airport you're connecting at and which airlines you're using.