The relevant figure here would be how long Spotify had been running, wouldn't it?

There's probably some way to get that info too, but of course I killed the app as soon as I saw what was going on, so it's too late!

Edit: uptime does give it a firm upper bound, at least. My current uptime is a bit over 12 days. So that's an average of 80GB/day at least, more if I started Spotify sometime after boot.