Our open mics at Folk At The Forum in Cobourg, Ontario stated 15 minutes or three songs, whichever came first. We had some very subtle MCs who could inform a player that his/her time was up without offending them, but this was seldom neccessary. Open stage players were asked what time slot they'd like and we tried to accomodate them - some folks were intimidated by going on first and others wanted to get it over with. We had an open mic before each show with a hired performer and one or two all night open mics a year.
I often asked people to perform at our venu whom I'd met at other venus without feeling guilty about "stealing their talent". I also often appeared at other venus myself. If you do start an event of your own, and I hope you do, you should feel free to invite anyone you wish. I'd try to steer clear of having it on the same night as the other event though.