Median follow up period since what? That's for you to choose. Median follow-up since entry into the study? Since randomisation? Since first treatment? Since discharge from hospital? You must choose what is scientifically and/or clinically interesting, and/or relevant. Once you've chosen what period you are interested in, the answer is clear: the median is the length of "follow-up" such that half your subjects were followed up for less than this length of time, half were followed up for more.