For a 10 year multiple entry Chinese tourist visa, how early can I apply?

You can apply any time but the 10-year visa may be hard to get. If you haven't traveled to China before, then a normal tourist L visa is more possible to get and applying one month before your travel is a recommended

Note that once your passport expires, the visa also becomes invalid. So if you still plan to apply for a long-term visa, make sure your passport have a long validity.

Margaret, if your passport expires prior to the expiration of your China Visa, you can carry your old passport with you with the China Visa along with your new unexpired passport. You present both passports to the immigration officer on arrival to China. They will accept your valid China Visa and allow entry. I've done this multiple times as I have a 10 year multiple entry visa for China as a US passport holder.