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):
Return to Ravnica
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.