Good question! This really depends on your personal usage, but you can use the table below as an estimate.

One email (text only)35 KB
One email with photo attachment1 MB
One email with Word, Excel or Power Point attachment300 KB
One webpage300 KB
One app/game/song15 MB
Social media download/upload500 KB
One 4 minute song600 KB
1 minute of streaming video (e.g. Youtube HQ)30 MB
  The above estimates are measured in KB and MB. If you're not sure what these are, see the table below.

1 Kilobyte (KB)1,024 Bytes
1 Megabyte (MB)1,024 Kilobytes
1 Gigabyte (GB)1,024 Megabytes
  When data is consumed, it is used in batches of 10KB (10,240 Bytes to be precise) at a time - these are called pulses. Your data consumption is measured in volume (Bytes) by the system and rounded to the nearest 10KB. This is then deducted from the data included in your plan.