There are many reasons why you can work on a picture for several hours on a single day :
- the paint needs some drying before you can apply the next layer
- YOU need a little rest and are desperate for some coffee;)
- after finishing for the day, you take another look at your picture (after an hour or so) and decide: no, that´s not enough!
To illustrate this, I´m posting two versions of the same picture, I´m currently working on, that were made on the same day.