AFAIK the world record for this is held by Moscow, where some lines run every 80 seconds in regular schedules operation. That's departure to departure, and it's virtually impossible to go below that, because the speed of humans boarding becomes the limiting factor. Some automated lines claim 75 seconds as a theoretical limit, but AFAIK nobody actually does this.

Intersting, I've just googled a bit, and Ligne 1 in Paris was running every 90 seconds in peak hours, most other lines are around 100-110.