What should be cycle time for any meditation technique?

Q: Pranam Gurumaa, I am connected to you since the last 5 years. I am 27 years old and I work with a legal processing firm as a Software Analyst. My work timings are from 9AM - 9PM. I reach home at 9 PM. With your grace, in the mornings, I practice a meditation technique (mostly pranav dhyana or waheguru dhyaan or mool mantra). 1). Is it correct to practice one particular meditation technique for a span of 40 days and then change to another meditation technique and then the cycle goes on? 2). As I reach home at 9 PM, I am unable to practice yoga nidra in the afternoon. Can I practice yoga nidra when I reach home at 9 PM after all days work? Please guide me accordingly and correct me if I am practicing the same in a wrong way. Gurumaa: No, I believe it one should practice one method for at least 6 months & secondly yes - you can do yoga nidra when you come back. But you must be very tried so when you will lie down to do the yoga nidra - my apprehension is that you would be just go in nidra & not in yoga nidra. But still no harm. Go ahead and do it.