Goldilocks

Too little, too big; too hot, too cold; too hard, too soft.....And then there's just right.

How do we know when to stop painting and call it done?  It starts with the concept.  If you are trying to duplicate a scene splinter for splinter, petal for petal, you will probably wind up with too many details and not enough clear shapes. Some painters go for the details way too early, and they end up with too much texture everywhere in the painting.  Other painters haul out the darks too early and have no midtones to tie shapes together.

One way to avoid these two pitfalls is to force yourself to stay with the biggest brush you have for as long as possible.  The minute you reach for a smaller brush, stop and ask yourself if it is indeed time to start with the details and textures and small shapes.

You can also question whether you are naming objects to be painted instead of shapes. "Now I'm going to paint the trim on the windows" can be replaced with "Have I defined all the major shapes and values in this painting?" Only then should you think about details.  If you begin with the details because you are impatient to get to the important stuff, you will nearly always end up with a scattering of textures that do little for the rights of the overall effect you want to achieve. 
As a fellow watercolorist says to his workshoppers, "First bake the cake; then you can decorate it."