If you paint in acrylics like I do half the time, to keep your brushes from drying with paint on them, after wiping the heavy paint off, dip them in water enough to get them wet then lay them down flat on your table or whatever you use to paint on. This will keep them moist until you are finished for the day and then you can wash them properly in soap and lukewarm water. If you leave your brushes in water they will eventually fail and fall apart. Give them a squirt of water from time to time.
On the other hand leaving them in oil or painting medium when painting in oils does not seem to harm them. I do it all the time and they can remain for days or weeks that way.
When I paint I have a habit of painting for maybe half an hour or so, then clean my brushes, then a while later go back to the painting again for another short time. I seldom stay at it from beginning to end. That way I give myself time to decide where I am going wrong and what the painting needs to look right. Art never comes easy. But when it does come through it lasts forever.
One should cover the canvas in each sitting. What I mean is to work on all areas and not concentrate on just one spot. This allows better colour harmony and helps avoid places on the canvas that look over worked.
