Where are official changes announced by Wizard?
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.