That setting is what Cloudflare passes on down to your visitors, not how long it caches itself. I don't actually know cloudflare personally, but assuming they'

That setting is what Cloudflare passes on down to your visitors, not how long it caches itself. I don't actually know cloudflare personally, but assuming they're following the spec, you need to set "Cache-Control: max-age=300" or "Cache-Control: s-maxage=300" to tell cloudflare to cache a given response for 5 minutes.