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.
I tried, but instead it told me I couldn't, and there's this message:
"We've seen some questions in our Community around the amount of written data using the Spotify client on desktop. These have been reviewed and any potential concerns have now been addressed in version 1.0.42, currently rolling out to all users."
So... sounds like it's fixed? If so, good job on making a fuss, everybody!