I think it will last a little while. There has been speculation, based on what Marvel tends to do with every popular book, that it will go until they can claim a Fantastic Four #600. So that would be less than a year.
@DalekDoctor2011: Can't say I agree with any of those besides possibly She Hulk.
@DalekDoctor2011: Dragon Man and Valeria are already on the team and I believe Franklin as well after Johnny's death.
The thing about this team is they are trying to still maintain that family dynamic. What they all have in common is that feeling. This isn't a team to have contrasts or struggles, but something to bond them together as a family force to make the world a better place. Of all those mentioned, none would fit in to that spirits (especially Namor!), but Beast and Lyra could possibly also. I just so happen to love Lyra, I wish She Hulks was an ongoing.
I do think they need one or two more "field" members to balance the amount of brains they have.