The final season comes out on DVD on March 28. If my memory is correct, HBO had an erratic release schedule for the sets. There be huge gaps but towards the end of the series, they sped up the releases.
Seasons 1 and 2 had something like an 18 month gap, season 3 came out maybe 1 year later I think... season 4 3 months after that, and now season 5 the following spring. Very weird pattern...