But go to far, and it can be a boondoggle. :-)
At the bottom of the article, you'll find the hard part:
When you click to edit your conversion window, you'll see a link to the Search Funnels time lag report. When you view this report, you’ll see the average and a distribution of the length of time it takes your customers to complete a conversion after clicking on keywords in your account. You can use this to help you decide how long of a window to set.