Black/oily growth, but no green: Clean every last bit of it off every three days.
Dark brown growth, but no green: Clean all of it off every 4 days.
Light brown growth, but no green: Clean most of if off every 5 days.
Mostly brown growth, but some green: Clean all of the brown off every 6 days, but leave some of the green.
Half brown and half green growth: Clean most of it off every 7 days.
Mostly green growth: clean most of it off every 10 days.
In all cases, stronger lighting will help you get to green faster, even if the extra lighting is only temporary.
Also, when there is lots of green, you are getting the most filtering. So to minimize nutrient spikes which might occur after cleaning the green off, consider cutting the screen vertically into two halves, and cleaning only one of the halves every 5 days. This give 10 days of growth for each half (about the max you can go), but always leaves the other half to do the filtering.
After several months, you may start getting some thick brown algae that feels like fuzz, and it won't come off. That's turf algae. You don't want turf because it's thick and dark, and it blocks light from reaching the screen (also, because turf does not grow fast enough to be a good filter.) Scrape the turf off with a hack saw blade. Matter of fact, if you start getting turf, you can just start doing all your weekly cleaning with a hack saw blade.