How do you spell "not for long"?
The adverb “briefly” is written in two versions: the negation particle “not” is combined with the adverb “for long” and separately, since “not for long” is an exception-rule word and has a double meaning.
Example: a stroller is needed for a short time. In this example, the adverb "briefly" is written together, as it is argued that the child needs a wheelchair for a short time. The same example, but with separate writing of the particle “not”: the child does not need a wheelchair for long. In this case, “not for long” means “not for a long time,” so the particle of denial “not” is written separately from the adverb.
How do you spell a short time if there is a union “a” in a sentence with this phrase? We write separately: not for long. Example: she came to us not for long, but only for five minutes.
To the same adverbs-exceptions, which can be written in two versions, include: shortly, inopportunely, not much, not far away.
Let's sum up. We write "briefly" together, if there is no opposition to the alliance "a" and if the word has a semantic meaning "for a short time."We write “not for long” separately, if there is an opposition with the union “a” and if the word has a semantic meaning: “not for a long time”.
Following the link: you can familiarize yourself with the rules for writing the particle “not” with adverbs, including examples of writing and using the word “not for long”.