I would say that those numbers are purely subjective. For a full-time pro, the numbers have to be higher than for a hobbyist just looking to make a few extra bucks.
If you are happy with the trend of your sales, then your images are successful! Don't listen to people telling you how much you need to make in order for it to be worthwhile... If you enjoy shooting (and editing, and keywording, and uploading!) photos, then you don't have to make as much to feel like a success.
If you're strictly business, then you'll need to have more downloads.