Turns out the answer is three games.
This might be the best and most descriptive video that Brendan Shanahan has issued in his tenure and time of doing these explanatory videos. As he details, Letang risked taking a big hit himself by crossing through the ice with the puck on his stick, but shouldn't have expected a hit to his head like that.
On the play, Letang suffered a broken nose and left a bloody pool on the ice. Shanahan has explained multiple times that any injury resulting from a hit will weigh into the decision. That was certainly a contributing factor here, too.
There were a lot of factors that Shanahan considered on this hit. Of the many, one was that the position of Letang's head didn't change significantly when he shot the puck and thus the onus being on Pacioretty to avoid making the head the principal point of contact.
Pacioretty will be unavailable for the team's California trip, with stops in Anaheim, San Jose and Los Angeles.