Ngondro is a complete practice and contains all that is needed to attain complete enlightenment. It is also the "Preliminaries" or a series of pre-requisite teachings that one must study and practice prior to proceeding onto Tsa Lung Tummo (Inner Heat Yoga) or Dzogchen (classes for year 3 and onwards).
It is possible to take Ngondro over a period of several years. A way this could be done is as follows:
Year 1: Study and practice on the Four Thoughts
Year 2: Study and practice Refuge and the Bodhisattva Vow, completing 100,000 prostrations
Year 3: Study and practice Varjasattva and Mandala
Year 4: Study and practice Guru Yoga and Phowa
Other divisions could also be followed when the practices are followed in order and maintained. This is usually the best way for those of us following a busy lay life in Western countries.
In Namdroling Monastery, Palyul's main monastery in South India, monks generally follow a full month's retreat learning the practices and then follow this with a 6-month retreat to complete those practices (Bum Nga) as 100,000 repetitions of each section is needed to consider Ngondro completed.
At the Palyul Retreat Center, students are permitted to proceed to the second year's Tsa Lung Tummo course and beyond *only* when they have completed all four weeks of the Ngondro retreat. (Completion of all repetitions is not mandated for entry to the classes, but it is assume one will want to obtain the realization from having completed all repetitions and will do those on one's own.)