a day - 1 a couple of days - 2 a few days - usually between 3-6 nothing specific, so it is generally more than 2 and less than a week a week - 7 days

This is not common language, and I would expect to see it more frequently in literature. It refers to a few days feeling like they lasted longer than they actually did. For example, if someone had a lot of things to do at work during the week they might say "it has been a long few days", meaning it has been exhausting and/or boring.

A "few" usually means somewhere around 3-5 in number. Each of those days could still be long or seem long.