General antonyms:
* Rare
* Infrequent
* Uncommon
* Occasional
* Sporadic
* Seldom
* Rarely
* Scarce
* Unfrequent
More specific antonyms depending on context:
* Infrequent (emphasizes the lack of frequency)
* Rare (emphasizes the low occurrence)
* Occasional (implies happening sometimes but not regularly)
* Sporadic (emphasizes the lack of pattern or regularity)
* Seldom (implies something happens very little)
* Rarely (implies something happens very infrequently)
The best antonym for "frequent" will depend on the specific context in which the word is used.