Battery life will depend on a bunch of factors, but you'll get a lot more than one roll. The AAs in the camera will vary with amount of flach and rewind usage, and the 9v on the sensor with the temperature. I'm sure you'll get a lot of posts with more experience than I've had, but 30 days plus is pretty much a given.
Using the RS PIR (no timer) I normally get about two months of use. I haven't had one completely dead in that amount of time but I normally replace it at the 2 month mark just to be sure the battery doesn't go dead just about the time a big buck shows up.
I find I have changed my batteries much more than needed. I have changed the 9volt about each month and I think they were still good. I wasn't having any problem and thinking back I guess I need to check them out rather than pitching them. MC.