Technically there isn't one one of significance. I mean, you have a limit for your original question, but then you can add MANY updates. As for answers, I've seen some super long ones and never heard or read of a word limit. The issue with looong questions or answers is that a lot of people won't bother to read them.
Very few knowledge seeking questions or questions seeking to take advantage of other people's experience, need to be novels. In fact, probably none do. Same goes for answers. It's not necessary to write super long answers, although I've been guilty of getting pretty wordy myself (not super long answers though).
Short answer to your question - with no word limit, most people would do what they already do, skip those questions/answers or only read the first line or so and respond to that.