Umihara Kawase also has a time limit of 30 minutes on the overall playthrough, which is known as the "30 Minute Rule". If this time limit is reached without clearing an Ending, the next exit cleared will lead to a field which leads straight to an Ending, rather than the Field that exit would normally lead to. There is one Field (28) that can only be reached in this manner. The 30 minute timer can be shortened to 5 minutes by pressing the Select Button 100 times at the Title Screen. - KawaseFan

The game will transport you to the ending Field that is closest (in terms of number) to whichever you are currently on, so keep that in mind.