I've read using for more than 2 weeks causes a 'smokey' flavour to be imparted.
I've got a RIS on woodchips at the moment that'll be getting cacao nibs for 2 weeks, so far my plan is to lightly break them up a little more, soak in some Glenfidditch 15yr (just as that's what I've got on hand) for 48hrs, then chuck the lot in for 2 weeks before bottling.
Going to do a little more research before I crack on with this though.
...I've also read people put them in for a few months, put them in 170F water to sanitise, roast in the over to sanitise. Not an exact science it would appear