Hi Mate,
I have had a nem for 2.5 months now, I feed mine raw prawn the size of half my pinkie finger nail almost every day to every second day! It loves it! In the two months mine has doubled in size, which I find weird as everyone posted above feeds there's much much less than I do.
I notice with mine all the tentacles shrivel up very thin if I haven't fed it recently, then I give it the prawn, it closes up and eats it, then it opens right up and spreads itself out with nice swollen tentacles and the little purple dots on the tips of the tentacles really brighten up.
I'll just add, if mine isn't hungry and I try to feed it, it just won't grab the food, or it will grab it and release it. Then Generally my fish will finish it off or I remove it
Good luck and take care!!