As others have mentioned the exposure time is usually many minutes and sometimes hours long. My night shots I shoot mostly Acros and expose anywhere from 3 to 15 minutes depending on the ambient light, usually at f/5.6 to f/11. I use Acros as its well known for handling reciprocity failure well. This one was about 11pm:

Portero Hill/China Basin, San Francisco
Contax RX
35/2.8 PC-Distagon
Fuji Acros in Diafine
by rich8155 (Richard Sintchak), on Flickr