They'll go as far as the league needs them to go. Notice that all of the NFL's relocation candidates are doing well the last few years. Buffalo, Cleveland, Tennessee. You could argue that Arizona is in that group as well. There have been rumblings that the NFL will be forced to give St. Louis a team again if they lose the lawsuit filed against them by the city.
On the other hand, the NFL might have already made its decision with certain teams, which is why they don't need them to do well. Jacksonville always seems to be on the chopping block and there are rumors that Washington will be the NFL's sacrifice, making the Ravens the de facto team for the DC media market.