When I make bourbon porters I typically soak oak chips in bourbon for a week and I then drain the chips and put the soaked chips at the bottom of a fermenter for secondary fermentation for an additional week to impart the bourbon flavours. So in total, primary fermentation about a week, then secondary with the chips for another week. Then bottle and leave the chips behind.
I can't see how using rum instead of bourbon would be any different?
Just make sure when you drain, you don't leave them to dry. They should still be wet, but you don't want your entire 750mls of rum going into the fermenter