John, How do you know when to harvest your sunflowers? I've got several, none as big as yours. WOW!!!
When I grew them in Denver's dry air, they were harvestable when the seed head itself was largely beige vs. green looking....I think that general rule will apply here. Plus you can always pry out a seed or two with a butterknife to sample for ripeness. I might have to cover the seed heads with some kind of mesh bag if I don't want my cardinals to get all the seeds. I will leave the smaller heads available to the birds. 'Mammoth' sure is a good name for this variety!