You earn one free week in the same category of suite you most cruise in for every 140 days. There are no limitations on the cruise you take, with the possible exception of Maiden Voyages. You vest after the cruise, so if you are close segmenting a long cruise so that the second or third leg is free is a possibility.
But also know that Seabourn is revamping its Past Passenger Program. I cannot say what the new benefits will be (they will probably be, knowing Seabourn, the best in the industry) or when it will roll out, but rest assured this one week for every 140 days of cruising shall remain.