I'd suggest keeping it as short as possible. It's your show, not Frank Zappa's. It's an intro, not a song.
A good one is the last 1:30 of Carmina Burana (
Youtube of Carmina Burana -skip ahead to 1:06:00 through 1:07:30.
Then again, if a 4 min. song works for your stage show, then go on with your bad selves....