It really depends on your personal needs.
We would use the following assumptions to calculate how much mobile internet data the following activities could approximately take:
| Download a webpage: | 100kB |
| Check your email: | approx. 50kB per email without attachments |
| Instant message: | 100kB per Instant Messenger session |
| Download a document: | 100kB per page |
| Download a song: | 5MB per music track |
| Download a photo: | 2MB per photo |
| Video call on Skype or Facetime: | 1.5MB per minute |
| Watch a video: | 5MB per minute of video watching |