I know I have to say "How hot is it?" instead of "How much hot is it?" (How + adjective).
But what if I have a comparative? Do I need "much", are they ok both ways, or do I have to leave it out?

It's deeper, but... How (much) deeper is it?
It's longer, but... How (much) longer is it?
She's older than you, but... How (much) older is she?

Thanks Emotion: smile

Yes, you need the much in those questions. It emphasises that you want a quantity as an answer.
Thanks Rose Emotion: smile