Sometimes never. Only if I'm not happy with the first draft.

Depending on what I'm writing, I edit at the end of every chapter. And sometimes, after I've already put the whole story to bed, I'll go back and read it - and find more mistakes than I realized. It's an ongoing process for me.

Depends on what I'm working on.

I suppose. Sometimes it's when I'm done, sometimes it's as I go, sometimes it's not until I get stuck and am trying to figure out where to go.

I usually re-read the past few pages after every couple paragraphs I write.

The first draft is for writing everything down as fast as you can.
Editing is to see if the story is really and truly for you, and is when you embrace it and make it yours.

I usually edit while I'm going through, and double check when I'm done.