A beginner should do wall sits for 30 seconds to 60 seconds in a set of 3. If you feel it is difficult to achieve, kindly reduce the time span but increase the number of repetitions from 2 to 5 sets. It is hard for a beginner to do wall sit for a couple of weeks, where you should hold for at least 30 seconds. But after 4 weeks increase the time of wall sits to 45-60 seconds.