Well, really the sooner the better. Humans should have started phasing out fossil fuels 50 years ago, so we are extremely late already, and the damage is already irreversible. So when do we get serious about ending fossil fuels? Very difficult question since our entire lifestyle and "civilization" depends on them. But every little change we can make to reduce consumption, even today or tomorrow will help. The Collapse is coming one way or another. All we can hope to do is reduce the most horrible effects of the huge transition that is coming. The sooner we start, the better.