The format rotation is pretty predictable (although subject to change at WOC discretion).
Cards allowed (aside from cards that get banned on an individual case basis):
- Cards from 2 most recent story-related blocks
- Cards from any Core set released after the older story-related block in standard.
Example (from May 13, 2014):
Current standard:
Return to Ravnica
Gatecrash
Dragon's Maze
Magic 2014
Theros
Born of the Gods
Journey Into Nyx (effective May 2, 2014)
Magic 2015 Core Set (effective July 28, 2014)
When the story-related block is released (after Magic 2015 Core Set) Return to Ravnica, Gatecrash, Dragon's Maze and Magic 2014 will cycle out.
A new Story-related block and its first set are usually released every Sept/Oct each year (2 new sets of the block follow after 3-4 months each). Core sets usually come out in June/July. Each story-related block is in 'Standard' for about 2 years. Later sets in block and the core sets usually cycle out in about a year.
In 2016, it is planned to change the release schedule of new cards and block rotation for the standard format. Standard will consist of the last 3 blocks and each block will rotate out after 18 months.
http://magic.wizards.com/en/articles/archive/mm/metamorphosis