I have two "small" sinus polyps of the each maxillary sinuses. When should I consider having surgery? What's the likelihood of recurrence?

See Immunologist. Nasal polyps should be evaluated for underlying potential causes (disease, medication effect, etc) and treated with medication or possibly even using procedures to decrease the activity of certain metabolic pathways; or by avoidance of contributing factors. An Allergist/Immunologist may be best equipped to perform this work-up. Polyps usually return after they have been removed surgically.
When symptomatic. Generally, asymptomatic sinus polyps can be left alone. We treat them if they become chronically infected, cause pain or created blockage.
Sinus Polyps. If the polyps are small and are not affecting your breathing they may not have to be removed. However, if they are large and obstructing the maxillary sinuses; and if you are experiencing breathing issues they should be evaluated fro removal. Please seek the advice through consultation with a periodontist or oral surgeon.
ENT doc. Depends on symptoms. Probably no treatment without symptoms or radiographic changes. Make sure you are followed up by ENT doc.