Each air freshener lasts at least 30 days assuming daily use of Roomba for 1 hour a day. Duration depends on usage time, temperature / humidity, and storage conditions. Typical fragrance duration is 30 - 60 days.
The fragrance substrate is EVA which is a polymer designed specifically to allow the fragrance to dissipate consistently over time. Common fragrance systems use paper loaded with oils which dissipate a high level of fragrance at the beginning and much less over time. RoomAroma's EVA is found on more expensive fragrance delivery systems and produces a high level of performance.