If online money received 6 times a year then what id ideal time for Gift deed

Dear Sir/ Madm, 

Please consider this use case and help me here -

Son who lives with his parents transfers 1 to 2 Lakh every alternate month to his mother as a gift.

1. When is the ideal time to create a GIft deed - say March mid as its FY end OR every time he transfer the amount to his mother?
2. Preparation of Gift deed - should it be notarized or any plain paper and signed by both will do?
3. Are witness needed for the gift deed? blood relatives or others?

Thanks in advance.