Aside from refereeing iPad turns between my two kids, the activity I find myself doing most on the iPad is reading newspapers and magazines. Maybe I'm a slow reader or constantly interrupted by said children, but often times I don't finish a page of an article before the iPad dims on me. (Unlike on the iPhone, you can read on the iPad without scrolling every few seconds.) I then have to tap the screen to return to the normal brightness setting. And if I fail to tap the dimmed display in time, the iPad goes dark, forcing me to swipe to unlock the iPad and return to the article I was attempting to read. Thankfully, there is an easy way to prevent the iPad from so rudely interrupting you mid-sentence.
In settings, under General, tap Auto-Lock. By default, it's set to 2 minutes (and it dims 15 seconds prior to locking). You can change the iPad to auto-lock every 5, 10, or 15 minutes of inactivity. Or you can just turn it off by selecting Never.
Even for slow or distracted readers like myself, 15 minutes ought to give you enough time to slowly read and absorb any article; setting the iPad to never auto-lock could drain your battery if you forget to close out of an app and leave it running.