Depends on a lot of factor:

If These are top notch opponents and that the guy hasn't been shaken up seriously in one of the fight, I say 3 times. If the guy fought top notch guys but got shaken up pretty bad, I say that it might be reasonable to then decrease to 2 times for the time being in a 12 months span. If these are tomato cans like what Chavez Jr used to fight galore, then I say 4-5 to stay tuned is reasonable. It all depends of the context, really, but in an ideal situation, 3 times is quite a good compromise.