I planted a few Dioscorea Elephantipes seeds 3 weeks ago together with some chili seeds. The chilis sprouted almost immediately but the Dioscoreas can take up to a year to sprout. I was happily surprised to see a seedling push through the substrate as I prepared breakfast for the kods this morning! No April's fool joke!
I would never do bonsai from seed, it's just to long waiting. Dioscoreas grow really slow as well. The caudex grows about a centimeter in diameter a year. I would rather have bought a 3-4 year old plant but I have been looking for one for three years now and realized I can just as well grow from seed. They are not very common in stores in my part of the world.
Here's a link to some older specimens:
Here's a picture of mine: ;)