Give yourself completely to whatever you like for three years -- let it be your career, business, health or any other thing of life. Why three years, exactly? Because what is the advantage of sticking to something which isn't giving any result. Time is precious, don't waste if the results aren't coming. Why not less than three years? If we spread these three years for success, we can assume first year is dominated by getting knowledge of the field, second year dominated by execution, and the third year comes the maturity where knowledge and experience of execution strike for success. As an exeption we can stretch these three years into five or six years -- only when there are uncontrollable issues like of health or circumstantial issues.
Nyas means to establish something (Devi / Devta energy) in our body: it purifies our body and makes our body and mind worthy of chanting a mantra / stotra. A Sadhak should do भूत-शुद्धि, बहिर्मातृका न्यास, अन्तर्मातृका न्यास, and षोढा-न्यास daily, 'coz we worship Devta by becoming a Devta, and in that Nyas helps us a lot. In every mantra generally many kinds of Nyas are given prior to chanting a mantra, they are usually shorter and doesn't take much time. They are essential and shouldn't be skipped. These भूत-शुद्धि, बहिर्मातृका न्यास, अन्तर्मातृका न्यास, and षोढा-न्यास take comparatively longer time duration. भूत-शुद्धि means purification of five elements in our body, that is Earth (Prithvi), Water (Jal), Fire (Agni), Air (Vayu), and Ether (Aakash). बहिर्मातृका Nyas and अन्तर्मातृका Nyas, establish "energy of our VarnMala" in our body-parts and in our chakras respectively. षोढा-न्यास is of two types Lagu Shoda Nyas and Maha Shoda Nyas.