What is the duration between a conference and its online proceedings?
Depending on the particulars of production on both the conference side and the IEEE side, you should expect anywhere from a few weeks to six months.  Both the IEEE and the conference staff can introduce delays here (I was recently publication chair for a conference whose proceedings were delayed by five months by the IEEE simply because they kept dropping the ball).

Note, however, that the IEEE allows authors to put a copy of their own work online.  This means that you can make your own work accessible much more quickly, and can often find PDFs of the works other others as well, well before the official version appears in IEEE Xplore.