You select a time period in minutes as input. It'll spit out how much storage it'll take up based on that + the selected quality (bitrate). I have selected 240 minutes and 24mb/s (highest available bitrate at this time), and the estimate it gave me was 40-45 ish gigs.

Edit: based on the OP screenshot, 60 min at 24mbs takes up 10.8gb:

60/10.8*500/60 ≈ 46.3 hours, nearly 2 days @ 500gb max storage.