Board Thread:Questions and Answers/@comment-25523075-20161019063650/@comment-28606847-20161019072130

A way to open the door for a longer period of time might be with replacing the buttons with pressure plates and/or levers.

If you want to do it with buttons, though, I think you can extend the time of the door being open with Redstone repeaters, and some kind of mechanism. I am not that great with Redstone, but if you'd like, I can try doing this and perhaps post a screenshot.