The headgear for my Mirage Activa seems far less durable than the headgear for my other masks, a Sleepweaver élan and a PR Optilife. It started looking fuzzy after only a month of use. It's at about six months of use now and I don't expect it to last any longer than your headgear. I can't judge the durability of the other masks' headgear since I wear the Activa the vast majority of the time.
I would imagine ResMed will tell you six months. I believe they are trying to make their product durability match the insurance reimbursement schedule.
I wish they would last longer too, but 11 months is pretty good. But if the headgear is so stretched out that you have to keep re-adjusting, and if the velcro stops working at about 6 months, then what have you accomplished by fighting with it to get it to stick? However, I don't blame you for trying to get all you can out of your headgear, ( milk it for all it's worth, Ha-Ha,) 'before you have to change it.
I mailed Resmed here in Sweden about the supposed length of life for the headgear.
This is the answer I got.
It was in Swedish but I ran it through Google Translate and it came out pretty accurat I think.
"We usually recommend that you replace the cushion and head band with approximately 12-18 months. I assume that you use the clips to on and off the mask? You do not adjust the straps so often then. The reason that Velcro wears out is just to open and close frequently".
I never use headgear for more than six months for precisely for the reasons you describe.