I don't really quit per se. Rather I pause and end up forgetting.

1. I don't have inking pens.

2. I lack the imagination/reference pictures to be able to go on

^^;;

Often it's the second.

Or...

3. Wait. I should go back to work/school now instead of drawing...

Though if I were to quit it would be possible that the pose and the idea isn't going so well but in the long run, usually I plan and then I end up with something else, I don't mind! As long as I have some good output I say!

i mostly stop at the pencil stage and move on
when i find i like an idea, i continue with it to the end