The bottom line is, we don't know. As has been said, there was talk of a sequel to Superman Returns for a while, then it became less and less frequent until it became apparent that it wouldn't happen. Much the same happened with Daredevil.
On the other hand, the original X-Men movie didn't originally look successful enough to get a sequel and even when it looked like it would get one, there was talk that Singer wouldn't be re-hired. Even Batman Begins didn't have a sequel greenlit right away. Few would have bet on a sequel to Ghost Rider but that's happening.
There's not an awful lot to be gained from looking at what happened with other franchises or even with previous movie incarnations of X-Men. Bottom line is - if Fox feel that First Class was successful enough to warrant a sequel and that the sequel will make them money, then there'll be one. If they don't - there won't.