I translated: What is often? This was not accepted. I think that often and frequent are synonyms, don't you?
In terms of usage, often is not really used any more as an adjective; it's an adverb.
A bizarre English sentence in any case - and I agree with Liesz: if you´re going to translate it ´often´ is as good as ´frequent´.
a very strange question in English, unless asking what 'frequent' actually means, or you have not heard what someone has said 'is frequent'. What circumstances would it be used in Greek?