At events and on conversation boards everyone is has stated that as Khans comes out M14 and the Return to Ravnica block is going to be rotating out of standard. I can't find any reason this would happen other than that "that is what happened in the past". As Magic has a very structured system I would imagine there is an official feed/site somewhere from Wizards that announces and clarifies such things.
WoTC has a tradition of reorganize/changing their webpage, so I cannot guarantee this link will work consistently in the future, but they generally post it on their webpage someplace (I found this using Google + "mtg standard format")
Standard format uses the two most recently-released blocks, as well as the current core set. When a new block is launched, the oldest block rotates out of the list. The following card sets are currently permitted in standard format:
Khans of Tarkir (effective September 26, 2014) Magic 2015 Journey into Nyx Born of the Gods Theros Magic 2014 (until September 26, 2014) Dragon's Maze (until September 26, 2014) Gatecrash (until September 26, 2014) Return to Ravnica (until September 26, 2014)
note: The quote in this answer is going to become outdated soon. Wizards has announced a change in how cards are released and how it will impact the standard format.
The list of sets legal in any format can be found in The MTG Tournament Rules, Section 6. The sets legal for Standard, in particular, can be found at this rules page. Historically, a block rotates out when the block after the following one is introduced, and a core set rotates out when the block after the following core set is introduced. However, Wizards is changing this, and in the future, core sets will be removed, blocks will be two sets each, and the most recent three blocks will be legal.