Should not take more than a month.
Some members getting real high scores is normal though - remember it sets targets based on your forums *average*. If you have a lot of barely active members, that will make the targets low, so your really active members will absolutely smash them.
Setting the threshold higher will help - that determines how many points someone needs to gain in order to be included as part of the average aggregate. For instance if you set it to 40, only people with 40 points per day (or week/month depending on what you choose) will be included in the target setting. So it would be Total points of people with over 40 / people with over 40 points, Giving you a much higher target score.
Hope that clears it up for ya ^.^