Anyone know how long approximately the mailbox batteries last?
Good question, @Dygme! The Mailbox Sensor requires 3 AAA-cell batteries (sold separately) for operation. As Ring Smart Lighting devices run on a power efficient, long-range RF Network, your battery usage is expected to be efficient depending, of course, on usage and environmental factors. I hope this helps! 
Is there a low battery notification that will alert of low batteries in the Mailbox Sensor?