It really depends where you're coming from and where you've been.
If your a former Prime Minister of an allied nation, you'll probably only be detained an hour[0].
Otherwise your mileage may vary[1]. Your local authorities can probably give you some guidance on this.
[0] https://www.washingtonpost.com/news/morning-mix/wp/2017/02/0...
[1] https://www.nytimes.com/2017/01/29/nyregion/airports-travele...