Monthly Traffic is dependent upon the what amount of data will transfer in a month we can calculate it day by also.
In general small or medium web site will consume between 1GB and 5GB of data transfer per month.
You can calculate this by multiplying your average page size with the number of expected page views per month. If your average page size is 30KB and you expect 50,000 page views per month, you will need 0.03MB x 50,000 = 1.5GB.
Larger, commercial sites often consume more than 100GB of monthly traffic.
Before you sign a contract with a host provider, make sure to check this:
What are the restrictions on monthly transfer data rate.
Will your site be closed if you exceed the volume.
Will you be billed a fortune if you exceed the volume.
Is upgrading a simple task