How Much Should You Write a Day?

Yesterday, I made a post about Tracking Your Writing Progress using a spreadsheet in an effort to develop good production habits. After I posted it, a friend wrote to me and asked how much I thought was a good number to aim for. It seems like such a simple thing, but it was an issue I struggled over myself, and still struggle over.

My initial answer to him was figure out whatever you think you can handle each day consistently, and then multiply that by 365 to get a target number. However, afte...