I just replaced mine for the first time, after using my R2A for 8 months. Not sure how many rolls I've shot but 40-50 sounds about right. It died rather suddenly. I noticed when I was trying to use the AE lock and it wouldn't hold it. I might have been able to get off a shot or two, but not many. Since they're so small, I keep an extra set (need to replenish) in my camera bag at all times.
Paul