I would add it towards the end, last 10 minutes or so. I'd worry about it carmelizing if it's in the boil too long. I'm not sure if carmelized lactose sugar would taste that good.
Did a sweet stout and added it at about 10 minutes left in the boil. Just remember to add slowly, that stuff sinks faster then DME and really scorches when it hits the bottom of the pot. In my experience.