Q:  What is the size of this drift in time?  Express it in both

           a) how long does it take for the signal to drift by 2 hours?


                The graph shows that the signal drifts by 4 hours
                over the course of 2 months, so that means 
                it drifts by 2 HOURS IN ONE MONTH.

                

           b) how many minutes does it drift per day?


                To a rough approximations, 2 hours = 120 minutes, 
                and 1 month = 30 days, so the signal drifts by
                about 4 MINUTES PER DAY.