The wet-on-wet technique is done alla prima or in one sitting. However, there are steps you can take to limit the amount of paint time. Have your canvas, prepped/gessoed and the imprimatura/stain and ink drawing complete. Then mix out all the colors and store in an airtight container. Use several brushes so you can clean at the end instead of during painting, or if you're doing a palette knife painting, have a box of tissues and a trashcan nearby. The paint should stay viscous, pliable up to 6 hours depending on mediums like Lukas paint butter or alkyd gel, some are fast dry and give you about 4 hours.