I noticed that some people write "awhile" together in sentences like this:
It's been awhile since ...
While most people seem to write "a while" as two words in sentences like this.
Also, according to this article, "a while" is the right way, because in this sentence "a while" could be replaced with a noun, not adverb: http://www.quickanddirtytips.com/education/grammar/while-versus-awhile
My question is, can you really write "it's been awhile since ..", or is it grammatically incorrect and you should write "a while" here instead?
UPDATE: This question is not a duplicate, as I'm asking about a specific sentence rather than awhile vs a while in general.