I am trying to grow Helianthus in my garden!  Have you ever grown them?
Yes I love them, I grow several types of sunflowers, I am an expert when it comes to them, anything you want to know about them? 
How long does it take for them to grow to full height?
Well I grow a special sunflower called "H. annuus", and it takes about 2 years if I'm not mistaken. 
Wow!  That is a long time!  can you eat their seeds?
Yes you can and If the husk is solid black color, the seeds are called black oil sunflower seeds. They are delicious when salted.