Gearbox Software has an extensive post-release plan in the works for Borderlands 2, with four DLC packs described as "content campaigns" arriving in the months after the game's September 18, 2012 release. The packs will all offer "several hours" of new material, according to the press release, including "new adventures, allies, enemies, and environments."
You'll be able to purchase each pack individually for $9.99 (or 800 MS Points) apiece, on all platforms. Alternatively, you can save a few bucks by nabbing yourself a Season Pass for $29.99 (2,400 MS Points), amounting to four packs for the price of three. There's no platform exclusivity for the add-on content, timed or otherwise, and Gearbox promises to have everything available for PlayStation 3, Xbox 360, and PC platforms by June 2013.
Also, to be clear: the previously announced Mechromancer pre-order bonus is not included as part of the Season Pass content.