There's no black and white definition. We have a guy who tried to contest a write-up for this, and I asked the same question. I think he missed something like 21 days since the beginning of the year. I would say any more than one a month as a personal rule.
To add on to this, do only unexcused call outs count? I have a coworker who got written up (she had called out maybe 5 days or so in the past month), but all of them were because she was sick, and my understanding is she had doctor's notes.