For users who are not logged in on their Spotify account, the Spotify Play button plays a 30-second long audio preview, and then the users are prompted to either login or to sign up, and they'll be re-directed to either the Web Player or the app. It's not possible to play a longer preview.
Hope that explains things!
Help others find this answer and click "Accept as Solution".
If you appreciate my answer, maybe give me a Like.