It is very simple - so you paid premium on Sept 8, and bill generation is on Sept 16.
You should most probably see the neu coins on Sept 19 or at the max sept 20th.
Now, how much was the premium amount? If it was 1000 rs, you should be expecting 5% = 50 rs total cashback.
Out of that you may get Rs 35/- on Sept 19. Remaining Rs 15/- will come on Oct 19 (after next bill).
(Or it can be reverse = 15 on Sept 19 and 35 on oct 19, i am not very sure.)