Answer:
Representatives are apportioned to the states on the basis of their population count as a percentage of the total US population at the end of a census year. The US Census conducts this count every ten years, and reapportionment occurs when states gain or lose a statistically significant number of residents (there is no fixed number). Changes become effective at the beginning of the next session of Congress.
The most recent census year was 2010; reapportionment of the House became effective in the first Session of the 112th Congress, in January 2011.