It took 2 weeks for those sunflowers to reach 5 inches, and 6-7 weeks to continously flower. The temperature here in Qatar reaches 48C during June-July and very humid in August. I started those seeds on August and it seemed to be doing fine under those conditions. I usually start growing the seeds in small plastic pots and when it reached 4 inches or so I relocate them to the ground. At this point they grow really fast and tall. If you leave them in the small pot for awhile they will grow but will produce really small flowers.
This is one of my favorite because it's easy to grow and It's a show by itself! I'm growing (Velvet Queen),(White Cream) and (Ring of fire) variety at the moment.