As you mentioned, the songs you play are around 7 minutes in length. I've noticed personally with the 30 minutes ad-free, it measures how many songs you play with the average length of each song on your queue. If you start playing another song, it replaces your queue until the song is over or if you add more songs to your queue. Whether you set a queue or not, Spotify automatically makes a queue for you to see what songs will be played. To view this, tap the song playing on the bottom of you app and click the three lines in the bottom right corner. You'll see Now Playing, and Next From: [Name Here]. If you have a queue that you've set, it will say something along the lines of Next in Queue. Spotify measures how many songs it will have to play before it can give you an ad. If you have a song that is long and next in queue that will go over the total time of 30 minutes, the ad will play first. Hope I helped. If you have any more questions, please just send over a reply! Don't forget to give me a vote if you were satisfied with your service.