It is well known that the Norse settlers in Normandy assimilated rather quickly, although a good deal loanwords were introduced into Norman French. What I haven't been able to find any source on, however, is when exactly Norse itself died out completely amongst the Normans.
Fair warning: I'm on cough syrup, and my mind is a bit hazy. I apologize for any errors of spelling, grammar, or formatting.
So I've just been through my library, and I'm afraid I don't have any firm answers to give you. This change occurred during one of the more poorly documented periods in medieval French history.
The various war bands of Danes, Norwegians, and Anglo-Scandinavians who settled among the inhabitants of Neustria and transformed that region into Normandy embraced Christianity and Frankish culture very quickly, within the first generation. Rolf, the first count of Rouen, had himself baptized sometime between 911 and 918 as a condition for the land grant. It's hard to tell how sincere he was in this, but in any event, his son, William, was a fully Christianized count. With Christianity came Latin as the language of scholarship and administration.
Herein lies the trouble: almost all medieval Norman writing is in Latin, while what we are concerned with is what people actually spoke. It's much harder to get an idea of how this changed and when. My guess - and it's only that, a guess - is that William Longsword was probably the last speaker. His son, Richard I, was raised by a Breton mother and had relatively limited contact with his father.