It's your choice. I know of some beekeepers who actuall requeen twice a year.
I generally only requeen when the queen is failing. I have one queen right now that is marked Yellow and that would make her at least three years old (3 1/2 actually). The studies show that a young queen is less likely to swarm. I have a lot of two and three year old queens and I don't really have a problem with them swarming. But I try to keep the brood nest open.
Of course if you use Apistan or Checkmite the queen will probably fail within a year and I would DEFINITELY requeen every year.
Oxaclic acid vapor doesn't seem to make any difference on queen longevity.