There's a slice of the population that needs accessibility devices to unlock their iDevices. It'd be unfortunate if they couldn't unlock their devices anymore due to a security feature. 7 days sounds like a compromise between usability and security.

Yeah, 7 days feels awfully long. When would you actually need that?

Edit: Okay, I suppose there might be an edge case where you break your screen and want to take a backup on an already paired device.

Well, why not 7 days?

The advantage of a smaller time period is obvious: if 7 days is secure, then 7 hours is more secure.

But presumably there's a good reason they chose that number, I'm just wondering why that might be.

It seems a reasonable default, but would be nice if there was a custom setting for a shorter time frame.

I assume it's a number they chose to strike a balance between security and eventualities that create support burdens.

It could probably be user-defined, but when it comes to setting a one-size-fits-all default and offering users an option, Apple usually selects the former.