But consider this. When students are meaningfully engaged in learning, it is not time wasted. Because art is experiential learning, and because they are doing things about themselves, children often have a much longer attention span for art lessons than for other studies. If so, use the art lesson as a way to develop their attention span capacities. If possible, budget time to teach the whole lesson.
Many teachers successfully continue one lesson over several sessions.
Think of it as a unit if that helps. This is a much better option than leaving out the meaningful parts. You can repeat the opening rituals to start each session, and include a short review before each session. If overall time is a real problem, consider scheduling the lessons less often rather than leaving out meaningful learning opportunities. The length of time we spend on each subject doesn't always make sense.