The ‘right’ time to desiccate sunflowers is when the back of the head looks banana yellow and the bracts are brown up to and past the shoulders. This visual combination equates to R9 or typically around 30% seed moisture. You want to visually check out different parts of your fields, looking at the areas that you know advance faster and the ones that advance the slowest. Try to aim that the majority of you field is at the R9 stage.
Reasons to desiccate would be that even though seed moisture is drying down, to properly thresh out the seeds, the head material and stalk material need to be dry as well. Desiccation speeds up time to harvest meaning seeds are off the field faster, which can mean less losses from head rot damage, stalk breakage and bird feeding. National Sunflower Association of Canada has just released a bulletin explaining more on timing and products at http://www.canadasunflower.com/
Not so sure on the visual description? You can hand shell out a couple of heads and test the seed moisture to see if the seed is around 30% moisture or less. An easy method is the microwave method, which is explained in this paper http://library.ndsu.edu/repository/bitstream/handle/10365/6109/farm_45_02_03.pdf?sequence=1
Submitted by: Anastasia Kubinec, MAFRD Oilseed Crop Specialist