During today’s Season 3 developer livestream, Blizzard laid out their plans for what both the upcoming season’s and future seasons’ balance update cycle will be like.
There will be two parts to a season’s updates, starting with its launch. The start of the season will bring the first of two class balancing passes. It will aim to buff under-performing builds and nerf the extreme outliers from the previous season. This part will also bring all the new seasonal content, including the new Uniques and Legendary Aspects.
We then have the mid-season patch, which will bring more new content (like the Abattoir of Zir or the Guntlet), as well as the second class tuning pass (which will be smaller and will only nerf explicitly bugged builds), other needed adjustments, and a return of some previous fan-favorite designs as items (the Malignant Rings, for example).
Blizzard also clarified some specifics on the possible adjustments in the second tuning pass. For example, the 
Violent Hammer of the Ancients Overpower bug that appeared in Season 2 would definitely be addressed, while the Necromancer’s 
Ossified Essence key passive being too powerful would not – that would get addressed in the next season’s opening balance pass.
And so, based on the above, we get the final roadmap:
There could be more changes to future seasons, of course, but this is the plan for the foreseeable future. Players get pretty upset when builds get messed with, so better to know when this might happen ahead of time. Hopefully this addresses that particular concern.