Mail at least monthly. A lot of people think monthly solicitation is too frequent. Maybe you don’t want to over mail donors and make them mad. I understand that.
Here’s the deal. In the months you aren’t mailing, plenty of other organizations are. It isn’t like their mailbox will be empty because you didn’t send them mail. And, you’re just increasing the chances they’ll be swayed to support another organization instead of you.
On top of that, unless you’ve got a crystal ball that lets you see into the future, you have no idea when each of your donors will be ready to give again. The best way to combat both of these issues is to be in the mailbox with a compelling reason for support each month.