How often do I need to clean my roof?

Well....that depends on many factors:

Method used: Cleaning with just water, no chemicals, last between 6 months and one year. Cleaning with Chlorine (H-Cl) lasts between 2 and 3 years.
Orientation: Depending on the amount of sun the roof receives it will determine how fast the mold can regrow.
Location: Areas with more trees, debris, or harsh weather conditions may require more frequent cleaning.
Roof material: Some materials, like metal, are more resistant to staining and require less frequent cleaning.
Age of the roof: Older roofs may benefit from more frequent cleaning to prevent further damage.