The Golden State Warriors are in the midst of one of those charmed NBA seasons that happen only so often. (from a WSJ article)
I cannot find the meaning of "only so often" here. Is this a idiom or should I separate, like only, so often??
Only so often is an idiom.
It means sometimes or occasionally with a slight emphasis on it being a rare occasion.
It is similar to every so often, which also means sometimes or occasionally but with a general feeling of being more common.
If I get my hair cut every three months, I might say:
Every so often I get my hair cut.
If my husband only shaves once every two years, I might say:
It's only so often I get to see my husband without a beard.