The traditional ruling on standard is the last two blocks plus the last base set.
Current standard:
Battle for Zendikar Block (Battle for Zendikar, Oath of the Gatewatch)
Khans Block (Khans of Tarkir, Fates Reforged, Dragons of Tarkir)
Magic Origins (base set)
With the new two set block
formation, the sets will change twice a year (Spring and fall)
When Shadows over Innistrad is released in April, we lose Khans and fates, but keep dragons and origins.
When the next block is released (I'm guessing in january) we lose Orgins and dragons. from that point it'll be three blocks before rotation.