First, seedlings of "Chocolate Silk Tree" (actually Albizia julibrissin 'Summer Chocolate') are NOT true-to-type. So, unless you are just experimenting to see what you'll get, I'm not sure why you would want to grow seeds from this plant. If you are wanting a chocolate mimosa, you'll probably need to buy one vegetatively propagated. The reason I say "buy one" is that the plant is patented and cannot be legally vegetatively propagated without permission from the patent holder.

Second, in your area, Albizia julibrissin if highly invasive (environmentally destructive). It is listed as a Category 1 (a serious pest), and is well documented to be doing some serious harm. Again, I would recommend finding something better to grow!

To answer your question about timing (for like if you were growing a more desirable plant), I would recommend either planting them out now where they would be protected from predation and letting nature take its course (stratifying them naturally, over winter), or, putting them in stratification now, keeping an eye on them (checking for any signs of mold and monitoring moisture levels), and keeping them in stratification until you were ready to plant them, next year.

I think I will try them just the same. I have already found a spot for them and am aware that they are invasive. The seeds that I purchased did state that not all seedlings would be chocolate. I will follow your advice on growing them and thanks.