It should be more than enough for a web site that is not excessive in size (amount of pages) or daily traffic.

i.e. if your site does use more than 40GB of transfer/bandwidth your site would have to be receiving a large amount of visitors on a daily basis and/or have many pages that are being accessed.

Example: Every time one of your web pages is viewed by someone on the internet, the size of that page goes towards your bandwidth usage. Below is an example of how many times a page would have to be viewed to reach 40GB of transfer

page size 30kb = 1,398,101 views
page size 40kb = 1,048,576 views
page size 50kb = 838,861 views
page size 60kb = 699,051 views

All activity on your account counts towards the bandwidth used for your account.

This includes :
emails incoming and outgoing (and attachments)
Uploading and downloading files
Page views
etc.