I've recently become interested in doing gesture and figure drawings to try to add more motion and fluidity in my characters as well as to learn more anatomy. However, I believe I add shading to early on in my drawings or to much detail in a short amount of time. What would be the appropriate time to add detail or shadows when doing figure drawing in class mode. Does anyone have any advice?
I think a better question is not "How much time is appropriate?", but rather "What is the goal of your drawing?".
Time isn't really relevant for how much to add if what you're adding doesn't meet your goal. A 30 second gesture isn't about creating a finished piece that is perfect in every way, its purpose is to be a warmup and create familiarity with the figure's construction. It doesn't matter if you spend 30 seconds or 10 minutes on the drawing if your goal is being met, time only matters to shift your boundaries to create new challenges. Timers are tools, not rules!
What I'm trying to say is, if your goal is add more motion and fluidity, will shading and adding details help? Or will it take away focus from creating fluid shapes and motion?
If you're looking for things to add and resort to shading, then you're already done drawing the gesture, and forcing yourself to fill up the full x minutes won't really help. What does help is to review your work with the time left, and see where your gesture can use improvement for motion and fluidity. Then take this knowledge and either fix your gesture, or apply it to your next one.
Shading and details are goals of their own. Trying to study 3 different things and limiting yourself to a timer is a recipe for disaster. Use the timer as a tool to push your boundaries and expand them. A member has mentioned that they gave themselves 10 or 15 minutes, and challenged themselves to draw a gesture in 10 pen strokes. That encouraged them to think carefully about the figure and what makes it a figure. 30 seconds does the same thing, but uses the time as the challenge, not the pen strokes, forcing the brain to recognize shapes faster. They have the same goal, but are executed differently.
So all I can say is, focus on one specific thing at a time and don't treat the timer like the end-all. It's useful in many ways, but if you're done with your gesture, you're done. That's always the time to review and either fix or move forward.
I agree with everything that's being said so far. I would add: Shading cannot rescue a drawing when the underlying proportions aren't right. So usually the answer to "when should I start shading?" is "When you have finished making corrections and are happy with the proportions, relations of features and limbs to one another, and confident the composition and where things will be is "done.""
The other answer is: When you are drawing for fun and not working on learning something new, start shading whenever it makes you happy. ;)