From the preliminary reports, it is sounding like Nomar Garciaparra suffered a groin injury very similar to what Mike Piazza suffered in 2003. A grade 3 groin strain (tear). As long as Nomar doesn't require surgery, then we can look to Piazza's injury as a guide to what awaits Nomar.
First of all, a nice summary of Piazza's injury can be found here. If you read that article and this one from ESPN you can see that they described Piazza's injury almost exactly the same way that Nomar's injury is being described today. Notice that while they call it a "Grade 3 strain," a Grade 3 strain is actually a tear.
Piazza went down on May 16, 2003. He returned on August 13, 2003. That's three months, after initially being described as "six to eight weeks."
Nomar is three years younger than Piazza was when he went down, so I think we can give Nomar an edge there as well as the fact that a shortstop doesn't rely on his groin as much as a catcher does. It was during this injury that the NY media really began to push for Piazza's move to first base. On the other hand, Garciaparra isn't exactly known as a quick healer, but then again, neither is Piazza.
So I think we are looking at, realistically, at least ten weeks without Nomar, and that the reports of "two to three months" should be more properly read as "likely three months." All of this is contingent upon Nomar not needing surgery. If he needs to go under the knife, he is probably out for the year, or at least until September.
The bright side of this is that when Piazza did come back, he didn't seem to have any trouble hitting the ball, going on an absolute tear in the month of August. He did slump a bit in September, but there isn't any indication that it was because of the injury. So when he does come back, since speed isn't really part of Nomar's game anymore, we can expect him to be pretty much 100%. The other bright side is that if his achilles is still bothering him, Nomar should have even more time to rest that up now too.
So that's what I can find out. If we're lucky, we should see Nomar back sometime shortly after the All-star break. That's not good news, but at least it's still half a season of Nomar if surgery is not necessary.