There is no meaningful averages. It depends on how much interest there is in the topic and how well you manage to get the word out so people know to look for it. In most cases it is a long, slow process.

There is a tipping point. When you start there is very little content because there are very few members. The low activity tends to keep new people from joining. Once you have enough people to maintain a reasonable level of posting it becomes a lot easier to keep it growing. Getting to that point can be frustrating. Just remember that it’s a marathon, not a sprint.