In my department we have three Oracle DBA's, most are DB2. Anyhow, we have a pager for production that we rotate weekly. We are a tight bunch and if we get stuck or need to switch it is never a problem.
I think monthly would be too much, but that is my opinion. Our Project Lead has the secondary pager but it never goes off.

You also need to evaluate your apps and how critical they are. Can some wait till morning? Communicate with them as well, nothing worse then having them load data without letting your department know ahead of time. Prevention is your friend.

When in doubt, SHOOT THE DEVELOPERS!!

MH