Duration of the flow
Ida and pingala function alternately for a period of one and a half hour at a stretch. Therefore, in a day of 24 hours the breath flows 12 hours in the Ida and 12 hours in the Pingala. This indicates normal functioning of the breath and good health. Generally the sushumna does not function when we are engaged in activities. When the breath is changing from one nostril to the other or when the mind is very calm free of all activities then the sushumna functions.

Natural Flow
On the first day of the bright fortnight that is after Amavasya, at the time of sunrise the flow starts with the Ida nadi. Thereafter, it regularly changes its course from Ida to Pingala and back from Pingala to Ida after every one and a half hour. This procedure continues for three days. On the fourth day the flow starts from Pingala and continues for next three days. Thus there is a change in the flow of breath on every fourth day morning. The reverse is the case with the dark fortnight that is after Poornima. The first day after Poornima on the dark  fortnight Pingala functions at sunrise. The summary of the flows are as under:

Ida
Flows at sunrise on the 1st, 2nd, 3rd; 7th, 8th, 9th, 13th,14th 15th of the bright half; and on the 4th, 5th, 6th; 10th, 11th, 12th of the dark half.

Pingala
Flows on the 1st, 2nd, 3rd; 7th, 8th, 9th, 13th,14th 15th of the dark half, and on the 4th, 5th 6th, 10th, 11th, 12th of the bright half.

Importance of the flow
Ida at sunrise and Pingala at sunset are auspicious.
A continuous flow of Ida through out the day and Pingala through the night makes one a great yogi.
Success ensures if the Ida flows on Monday, Wednesday, Thursday and Fridays of the bright half and the Pingala on Sundays, Tuesdays and Saturdays of the dark fortnight

Changing the flow
The flow of the breath can be changed by any of the following means from one nostril to the other.
Plug one of the nostril with a light cotton pad. The breath will flow through the other.
If you lie on the left side the flow will change to the right nostril.
The mind can be trained to direct and change the flow.

Changing to Sushumna
Lie flat on the back, no pillow. Concentrate on the tip of the nose or on the eye brow center. 
Sushumna will flow.
Sit in any meditation postures. Practice a few rounds of Bhastrika Pranayama. Concentrate as above.
It is the steadiness of the posture and intensity of the concentration that helps to maintain