Amount of Gb's depends of quality on your settings and platform. Some platforms support 320 kbps, some just 160 kbps. Of course there is always 96 kbps variant. When you listen, some tracks will go to cache. If you have Premium, you can also download your playlist(s) to Offline mode up to 3,333 tracks and enjoy these even without internet.
To download/stream 1 minute of song in 320 kbps, you need 2.4 MB, in 160 kbps 1.2 MB and in 96 kbps 0.72 MB.