The more inspired your sim is the faster they learn and the higher the chance of producing a higher level painting, so ideally you want to try and get your sim very inspired as often as possible.
You will want your sim to be inspired or very inspired whenever they paint, as it will not only boost the speed in which they learn the painting skill, but increase their chances of painting a painting of higher quality. Start 3 Paintings while Inspired (for 75 Satisfaction Points (SP)).This is a step by step guide on completing the aspiration if you would like advice on choosing traits and setting up to best complete the aspiration please read the previous post here.