Bornakk announced over at the forums that Arena Season 5 is nearing its end. He notes that the current season is slated to end "with or shortly after" Patch 3.1 is released. Season 5 kicked off in December 16 last year and has run around 12 weeks to date. In comparison, the shortest Arena Season was the pre-Wrath Season 4, which lasted for 17 weeks while the extremely lengthy Season 3 dragged on for 29. Season 1 lasted for 20 weeks and Season 2 lasted for 23, so all seasons thus far except for the Wrath-preempted Season 4 have run for a good length of time.
The reason for the extended Season 3 was because Blizzard had wanted PvP progression to match PvE progression, so were waiting for more guilds to gear up with Sunwell-level items. Considering how difficult Sunwell Plateau was, this took some time, delaying Season 4 considerably. PvE content in Wrath, on the other hand, was so accessible that by the time Season 5 started a month after release of the expansion, players were decked out in the best PvE gear -- which changed the balance of Arena play for the first few weeks.
Blizzard is looking to address that by starting Season 6 simultaneously with the "first tier of raid content" or essentially make PvP gear more accessible to prepare for the next season of Arena play, according to Ghostcrawler. Although Blizzard hasn't given a starting date for either Season 6 or Patch 3.1, the new Arena seasons are looking to match the shortened Season 4 in length and might indicate a trend in PvP season progression. That is an exciting change of pace and a comforting message to PvP enthusiasts that Blizzard is committed to keeping PvE and PvP progression at a similar pace. And if the development of Ulduar is any indication, that seems to be a fast one.