Soy wax is great, but it can be finicky to work with. I personally love it. Depending on the soy wax you are using, you can use up to 12% fragrance load. 1oz per 16oz of soy wax is the general rule to start with. You'll want to test it out and try going up or down from there to find the amount that works best for you and that particular fragrance. Some fragrances throw excellent in soy and some don't.
I do have a list on my website (in my signature) of fragrances I've personally used that throw well in soy. The burn time of soy wax is usually twice as long as paraffin, so I'm not sure that there is anything to add to make it longer. That's really going to depend on getting the wicking down right so that it burns as long as possible while still melting all the way to the edge and getting a good melt pool. You could try adding beeswax, but since I've never burned a beeswax candle, I don't know if it has any longer burn time than soy.