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 just Ninety-Nine Dollars.