A question I hear frequently from my students is, "How do I know when the sketch is done?"
It's a good question because a sketch can always be added to and improved upon. But how do we know when to put the pencil down?
The answer lies in beginning with the end.
Before you pick up the pencil to begin sketching, define for yourself what finished looks like.
Ask yourself, "What am I trying to accomplish with this sketch?" and then, "What does my sketch need to look like to accomplish that?"
Write these two answers on a sticky note and stick it somewhere visible while you sketch.
As you work, keep glancing over to your sticky note to remind yourself of your objective and stop once you've accomplished it.
p.s. this trick works for almost everything, not just sketching.