Is the world ready for this? A film of eight-and-a-half hours with Richard Wagner as the subject? Yes, we know, in the right hands the overture to ''Tannhauser'' alone can seem that long. But there is light at the end of the tunnel. There will be several intermissions in the Tony Palmer film due in London next month. Should reservations for orange squash be made early? Anyway, it sounds like a good warmup for the '84 Olympics in Los Angeles for which director Robert Wilson has promised a 12-hour stage play on the topic of civil war.