by Tom Lehmann
It depends on what parts of OTB you use. The petri dishes are mostly just nice bling, though they may save you a few seconds during setup.If you use just the new roles and events, almost no extra time is needed.
The Virulent Strains and Mutation challenges tend to add some extra time (maybe 15% more?), as they make the game harder (and therefore longer).
Ditto for the Legendary Epidemic card (though if you are going to lose, you will often lose *faster* when playing at Legendary).
The Bio-Terrorist challenge adds the most time and, indeed, is explicitly not recommended for 5 players, due to the added downtime that non-BT players will experience if they attempt this.