I have thousands and thousands of e-mails not only in my inbox, but in many folders….
It is difficult to give you rules on when you should purge your e-mails. But the first thing is to spend time getting all of those e-mails out of your inbox, and into action folders or into reference folders.
The second thing I suggest you do is to go into your folders, sort your mail by date, and start with the oldest, and just start purging. Another way to sort is to sort by those with attachments, because some attachments can be quite large, and can impact the performance of your system.
Another suggestion is to save the attachment on an e-mail to your hard drive, the first time you view it, and make sure that the e-mail references the path where the item is saved.
Finally, every time you are in a folder, take a quick look at what is “hanging” in the folder. You might be able to easily delete the out of date items in a few quick seconds.