The amount of money you make online is up to you. There are thousands upon thousands of "untapped" niches on the Internet. These are niches that contain an audience of eager buyers, and little to no competition. It's virtually impossible for these niches to "dry up" anytime in the near future, if ever.
Once you learn how niche marketing works, you can enter these niches and extract profits quickly and with relative ease, and you can do so over and over again, as many times as you wish.