During a standard comedy club show the emcee will perform 10-15 minutes, the feature will perform 25-30 minutes, and the headliner usually goes from 45 to an hour. In between acts an emcee usually needs to do a few announcements for the club as well as other various promos. (I wrote an entire giant chapter on everything that goes into emceeing a show in Don’t Wear Shorts on Stage.)
The big decision an emcee needs to make is how quickly to bring the headliner up after the feature. Sometimes a lot of the announcements can all be done after the headliner, so there isn’t always a lot to say. However, there are times when there should be a bit of a buffer. If the feature bombs, it might help to do a minute or two of material to bring the crowd back into it; you could even check for birthdays, etc. If you’re out of material, you can even try a newer joke and if it flops, make a joke of that. You should have at least one trusty line saved up that you can get a laugh.
If the feature has a great set, you can even ask the headliner (if he or she is nearby) whether they want you to bring them right up, or let the crowd settle back down. I always prefer to keep riding the momentum of the comic before me in whatever format of show I’m in. Some headliners have a more subtle beginning and may want the crowd to calm down so that it doesn’t feel like they’ve been buried.
Another thing to watch for is when half the crowd gets up to smoke or use the restroom. That’s when you should definitely stretch the show a little bit (provided it’s running on time and there’s not a second or third show scheduled that night) and take your time on the announcements. No headliner likes to take the stage to a half-empty room. The opening joke is so important so if a lot of people miss it, it can be detrimental to the set.
And a final message to headliners: Please be somewhere in the vicinity of the emcee so they can ask you what to do. No one likes taking the stage not knowing where in the hell the headliner is because he/she feels too cool to stand near you for two minutes.