It is difficult to advise in this matter, except to say that this is one of the most important techniques of yoga. The longer and more deeply you practice any technique, the sooner you will become proficient in it. It is for you to decide how long, in proportion to other techniques, you want to watch the breath. Regardless what techniques are practiced, however, at least the last quarter of one’s meditation time should be devoted to simple meditation, without any practice of techniques. As my guru put it, intuition (which he defined as the soul’s power to know God) is developed by prolonging and deepening the peaceful after-effects of one’s practice of the meditation techniques.