It depends on individual Users.
A typical MP3 file 3 mins - 4MB (estm)
A typical DVDRip file 3/4hour - 600MB (estm)
A typical BRRip file(Mkv) - Anywhere from 1GB - 12GB(estm)
All are just estimation to give rough idea of what to look out for.
You could actually gauge your usage using several apps.
Please do read the disclaimers, as most/all apps will not be able to give you a 100% reading.