Can you clarify what patches, and are you talking on the server or client side there is allot of variables in this. The server will only store the patches that you download so in essence if you download every patch for every solution it will of course require more than say the patches for office 2010.
It really depenends on the number of vendors, vendors' products and languages selected for download. It may vary from 100 Mbytes to 10 GB per month and more.
What you can do is: Go to Remediation Center and download all updates for given month for all product of your interest. Once the download task is complete go the download location set in Settings > Software > Patch Management > Core Services
Sort the folder view by modified date , select recently downloaded packages and check the size in Properties. This gives you the roughly the ideas how much space you need every month.
Bear in mind it still may vary from month to month. For example the Service pack are usually significant size. I hope it helps