I would have to agree with Vishalvasu….
309G is quite a bit of space your are sucking up….part and partial one of your biggest challenges you will face is to comply with local governance. As a rule, e-mail communication should be kept for a 5 year period…this could be different from country to country.

Your problem is not backing up that storage….the fact is that when you are going to restore is going to really be a problem. Exchange is a bit of a two sided coin…what to do and what not to do.
In my opinion I would first start by enforcing a storage limit as it might be that users are really going nuts.
You have to have mail policy that outlines your objective in how mail storage will be managed.
My policy that I enforce is that I will only allow MS office docs. Limitation per mail is set to 10 MB.
Further more, I also focus on archiving. Any mail older than 1 year is archived and then removed from the mail server.

My total storage went from 200G to 34G after enforcing that policy. My backup time is currently 45min.

I do a full backup every evening of my exchange server with GRT. My reason for doing this is that I am able to do single mail restoration per user and do not have to restore the entire database which will affect all other users.

I am not too jacked up on your application that you are using… I am using Veritas.

What is your typical throughput that you are getting on your backup job?
Pending on your kit…..2GB/min should be more or less what you can be happy about…

Hope this helps you a bit…

Cheers.

Vince