Apparently that's the ideal timeframe for a movie sequel to come out after the original or previous installment, at least according to Vulture. The original X-Men trilogy, the Austin Powers films, and the Bourne trilogy all saw their sequels improve in box office revenue, and all came out during the "sweet spot". Of course, the Ocean's trilogy was an exception, but for the most part 2-4 years between franchise installments benefited the films.
The analysis goes on to say that although it's a quick fix, yearly installments usually don't bode well for a movie's box office. After peaking with Saw II, the other 5 films in the franchise declined in revenue. Paranormal Activity has done well, but each sequel failed to match the original.
And although it has become fairly common, the worst thing for sequels is coming out 6 years later. The American Pie, Scream, Rush Hour, Meet the Parents, and Spy Kids franchises all had less-than-stellar sequels after much too time off. We'll see, but so far MIB 3 hasn't performed as well as the first two after a 10 year absence. Read more at the link above.