Depending on how much hazelnut flavor you want, you could start with as little as half a pound to about a pound or so. My gut says to put it as close to the end of the boil as possible. What you may even want to do is crush them, roast them in the oven (~400 F should do the trick) on foil, then add them to secondary or when active fermentation has stopped, treat the addition like a dry hop. I say this because the volatile oils may be lost from excessive boiling, and you want to be roasting as recently as possible. Roasting also (should) sanitize, so you shouldn't have to worry about infections, so long as you roast long enough.
You could also split it, do a third with the mash, a third in the last 15, and a third roasted and dry hopped.
One other thing to think about, that I just realized is that with the nuts comes a lot of oil, and that oil could kill head retention. Consider adding some carapils or a hefty amount of crystal to the bill.