If I'm interested in the topic and have something to say, if the discussion is active, then I see no reason to count number of posts. I guess it was over 50 a day when I was passionate. But usually it is not what will happen every day. If one day I post tens of posts usually I feel myself tired of posting and make much less next. So there is no average posts amount, it depends on topics I read in exact day.
As long as users are making quality posts then I don't think quantity is much of a concern even if they are creating them every 5 minutes per post. I never expected that users would be so guilty when they can make 50 posts per day with good discussion quality. Even if the campaign manager limits the 4-5 posts per day they pay or count, there is no reason why they should stop you from posting more than the amount they pay per day or per week.
If there is a signature I can be a little more motivated, but I feel myself much more comfortable with campaigns without minimum quota as I don't like to feel that posting should be done.
Of course, many people would feel the same way about that.