It took me 4 to 5 weeks of slowly mixing DS's old formula with the Nutramigen to get him to take a full bottle of it. He was a bit older (18 weeks) and so had some more developed taste buds I think, but we didn't push it too much - started with a 6 ounce bottle, 4 ounces of his previous milk + 2 ounces Nutramigen, then every 4 or 5 days increased the Nutramigen by an ounce.
Once he was on full bottles of Nutramigen it took about 3 days before we noticed some good changes in his behaviour, mood, sleep, etc. Then even 2 or 3 weeks later he was still having really bad tummy pains and thrusting around at his abdomen - paed said this was his digestive system "healing" from the damage the milk did and just to let it run its course. That lasted until about 2 months after he had started on full bottles of Nutramigen. From then on, all was fine.
So, for us it was a long drawn out battle but in the end, got there, and worth it!
Good luck, let us know how you get on!