One should always consider time when planning the events that will take place at your reception.
The average wedding reception is five and a half hours. After cocktails and dinner you should have three and a half hours of entertainment. If you decide to have karaoke I would suggest limiting it to an hour or less.
Amongst your guests you can probably think of one or two people at least that will get the crowd going. They can be told ahead of time that you would like them to sing the first song.
ASAP Entertainment can add karaoke to any regular wedding package for as little as $99.