A couple of weeks, unless you only have your tank light on for a few hours a day.
Only experienced green spot algae, apparently it is the sign of a healthy tank, but looks unsightly.
Live plants help to get rid of algae growth, as they take up the Nitrates produced by nitrifying bacteria during the cycling process, which is what algae thrives on.
Also Phosphates in the water supply contribute towards algae growth.
As live plants need around 10 hours of light per day, it can be a bit of a catch 22 situation trying to keep plants healthy and algae growth under control, so putting a timer on your tank light and providing a few hours 'siesta' period mid afternoon helps prevents algae growth, and keeps the plants thriving.