How long can you safely use Flonase for?

Flonase is a nasal spray containing the corticosteriod fluticasone. It prevents the release of substances in the body that cause inflammation. Flonase is used to treat nasal symptoms such as congestion, sneezing, and runny nose caused by seasonal or year-round allergies

Flonase is safe to use long term (life-long) but is more effective if used daily, as you point out.
However, some patients use it as you describe, and if it works for you then continue to do so.