I know there is no right answers but for learning purposes, which do you feel is better. Working weeks on a finished piece or doing lots quick pics. I am begining to think lots of unfinished quick pics are better. Just curious on what people think.
It depends on the study's purpose and what you want to learn from it. If you want to improve the gestures of your figures, do gestures. If you want to improve your color, do color studies. If you want to improve your 'finish', then do more finished stuff. Just doing studies in one area will help you in that specific area but will typically lead to a lop-sided skill-set where you're good at part of the process (like building up the image and planning it out) but bad at the rest (fixing it up, pulling it together, and finishing it off).
I always mean to strike a balance with my personal paintings, doing lots of speeds and thumbs and choosing the best to work up, but what usually happens is half an hour into a speed-paint I get into the 'zone' and I think THIS IS THE ONE! So I carry on, and then after another half an hour I realise how flawed it was and how I should have done a few more studies before trying to polish it off... and then I put it aside and can't find the energy for it. Or I roughly finish it, and dislike the result. So it's probably better to be more structured and make sure to get the thumbnail and speedpaint side of things good enough before trying to render something in super detail. That said, you can learn plenty from your mistakes so I guess there's no right or wrong way.
First of all, what do you mean by quick? 5 seconds? one minute? one hour? 5 hours? The purpose of lots of quick drawings/paintings is that you spend a lot of time solving a lot of different problems.
Working on a painting for a long time teaches you how to correct your mistakes and also teaches you when a drawing/painting is actually finished (this is actually harder than it sounds). I have ruined lots of paintings have been ruined by overworking the piece or they haven't reached their potential just because I haven't gone far enough.
I prefer to work on one long painting and, while I'm working with that, intersperse quick studies of features that are present in the longer painting.
In addition to the good points made here, let me stress that quick should never be sloppy, hurried or superficial. We can learn a lot from quick colour studies, quick gestures, but these all have a purpose, that should be achieved in short time. Quick or finished should both be deliberate practice...
I believe it's more about setting goals of what you want to accomplish in particular works/studies rather than how much time you want to spend on it. Though giving yourself deadlines is also a beneficial habit.