Touch the tip of the thumb to the tip of the index finger, with the other three fingers stretched out.
As it is the mudra of knowledge, it enhances the knowledge. The tip of thumb has centers of pituitary and endocrine glands. When we press these centers by index finger the two glands work actively.
There is no particular time duration for this mudra. You can practice by sitting, standing or lying on bed whenever and wherever you have time.