How long do batteries typically last in electronic deadbolts?
I've used something similar on my front door for about 4 years now (4xAA instead of a 9V) and it's great not having to carry keys everywhere. On average, I change the batteries about every 12-18 months. I guess it all depends on how often you use it and how worried you are about getting locked out.  Personally I don't bother testing the batteries because mine beeps angrily when they start to get low.