In my zone, I should be sowing seed around mid-August for a fall crop. Since you're much warmer than I am, you may be planting later. I think there's a thread in the veg forum about daikons and Farmerdill gave me some good guidelines. If I remember correctly, sow seed 6 weeks before last frost and I think you'd want to harvest before frost/freezes. In some of the permaculture references, some folks leave them in place to decompose in the ground, breaking up hard/unamended soil and adding some nutrients back to that lower layer. I think Rick C does sow them in spring, allowing them to flower and produce seed for future sowing.