Whatever you can get away with.
I've always considered the cave paintings found in France and other parts of Europe to be the finest art ever produced by man. They are realistic and impressionistic at the same time. They are unpretentious, honest, and demonstrate the commitment of artists who never sought fame or fortune, using crude materials that they had to hand. They literally breathe with life even after tens of thousands of years.
My own piece is simply a commentary on what a post-apocalyptic artist might consider to be representative of his life and times
Cave painting 2050