I usually do Cacao Nibs in a vodka soak, but I think they will be fine in the beer. I would start sampling after about a week, and then do it every 2-3 days until it tastes right. I don't know for sure, but if you wait for a month, you may end up with too much flavor, or an off flavor that develops. I don't think you need to worry about them being in a bag, they will sink to the bottom, and you can rack off the top of them. I like cacao nibs in dark beers, but I find the more robust the beer flavor, the more nibs I need. I use 2oz in a mild, but 4oz in a robust Porter.