Folklore Themes: Time Limit

In a story in which characters must accomplish specific tasks, they usually must do so within a specific short time frame. This time limit adds narrative tension, and also limits distractions and sidetracks in the tale. The time limit is set most commonly in a multiple of five days. That same factor appears with suspicious regularity in stories relating real history, where the actual duration of events are unknown or unimportant.

From Finding Old Ancestor:
Together we made the plan. I would go, with four others, and the rest would wait for ten days. We five would go five days and come back in five more, with water or with Old Ancestor.