How often does the standard set change?
Standard rotates once a year, in the fall. New cards printed in a core set or expansion set stay in Standard for between one to two years after their printing.

Currently, the normal set release schedule is:


One block (like Theros) on a yearly cycle: one sets each for fall, winter, and spring.
One core set (like M14) each summer.
A couple of specialty products throughout the year, like Commander decks or Modern Masters. These aren't part of Standard.


Both the structure of Standard and the release schedule have varied over time, but there's no particular reason to think Wizards of the Coast will change them anytime soon.

Modern-day Standard environments consist of:


The current block.
The previous complete block.
Any core sets released since the previous complete block.


The cards in Standard rotate once a year, when the first set of the new block is released in the fall. At its smallest, Standard is one old block + one set from the new block + one core set. At its largest, just before rotation, Standard is two complete blocks + two core sets. Because rotation happens in the fall, the cards in the summer core sets effectively "age out" the fastest (after about 1 1/4 years).