Eugene, OR
There's no fixed schedule -- it depends on your roof, how much shade it gets, and the weather that year. A whole-roof treatment after removal keeps spores from re-establishing, and annual maintenance plans exist specifically because moss tends to come back on shaded roofs without ongoing attention.
After manual removal, a professional-grade treatment goes on the whole roof. Treated moss turns white within about a week and washes away with the rain over 60 to 90 days.
A roof under heavy tree cover will re-grow moss faster than one in full sun, which is the main reason some homeowners opt for an annual maintenance plan with a moss-free guarantee instead of one-off treatments.
