infrequently has no recorded synonym. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. Its one recorded opposite is frequently.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
not many times
“in your 1850 church you not infrequently find a dramatic contrast between the sumptuous appointments of the building itself and the inhuman barrack-like living conditions in the church room”
Synonyms, antonyms and the broader and narrower relations for infrequently come from Princeton University’s WordNet. Full sources and licences.