I think that it is different from tank to tank as there are a lot of factors involved. lighting and photoperiod to start, you will get faster growth with more wattage and a longer photoperiod. If you have a large clean up crew, the algae will be consumed before it has a chance to "purple up". I have found that if you keep up on your Iodine in combination with at least a ten hour photoperiod with medium lighting and good flow, it could take upwards of a year to completely cover your rocks and the glass. Like I said, it is diffent for everyone. TinMan