Two hundred and sixty feet is, ultimately, the ceiling—but it is a pretty good bet that without some incomprehensibly large ocean engineering project to forestall the rise we will get there eventually.
Greenhouse gases simply work on too long a timescale to avoid it, though what kind of human civilization will be around to see that flooded planet is very much to be determined. Of course, the scariest variable is how quickly that flood will come. Perhaps it will be a thousand years, but perhaps much sooner. More than 600 million people live within thirty feet of sea level today.
