I read on two different sites that pixel mapping should be done once or twice a year. But sometimes I end up doing it once a month. Will it have adverse effect on the CMOS image sensor?
I can't imagine why. The actual operation is non-destructive — it just reads from the sensor when it should be dark (possibly multiple times), and records what is stuck on. Then that's put in a table — a map — which is used to mask out those pixels later.
Updating that map can't really be destructive either. I mean, it's probably stored on flash which can only have 100,000 writes or so, but that'd only be a problem if you were doing it every second, not every month.
I think the advice for once or twice a year is because new defective pixels can crop up, but it's unusual, and shouldn't happen frequently. Running yearly or half-yearly should catch them on average. Of course, if you notice one before then, running the mapping right away is clearly the thing to do.