Here is how you can approximate the time that your battery will last. A battery has a rating in amp-hours. That is in units of amperes times hours. Take the rating of your battery and divide by the current. Make sure your units are consistent. If you are using amp hours divide by the current in amperes not milliamps. One thousand milliamps equals 1 ampere. You then have your time in hours and you can divide that by 24 to get the number of days that your battery will last.
i haven't had a problem with the 9 volt battery and the RS PIR. The problem I have is the OWL's batteries. Just got my cam from the woods tonight expecting 24 pics and only 6 were taken. Started testing and found the camera batteries dead. They only had about 24 flash snaps on them. It must be the 90 deg + days we have been having. I am going to change them everytime from now on. The 9 volt battery for the PIR seems to last a good while for me. I haven't run one from new to no charge so I can't give you an answer as for as days. I change it every 2-3 weeks.
I put out my first cam on June 7th, and at present the 9 volt battery is down to 8 volts. That is all I expect to get out of one battery. I used the regular Duracell 9 volt Battery that cost $2 at COSTCO. I have 2 ea of those batteries inside the cam, but one is to power the light sensor.
I agree with possum, I change the batteries in the owl three times as much for thr RS pir. The 9 volt, I change once every two months. Depending on how good your batteries are. Believe it or not I got batteries from wal-mart once that were aready dead had to take them back. Check your expiration date just to be on the safe side.