First, I am a little strange -- I stock up big time on regular supplies. I have sealed buckets and other storage containers. Right now I have about 50 lbs of dry cat food and 150 lbs of cat litter (all the same brands) just for a single cat (Nano). I buy in bulk and on sale with coupons for convenience.
As for your situation, I would wait until their average age is 12 months and then buy the first big bag of adult food. I would make a first mix of 2/3 kitten and 1/3 adult and a second mix of 1/3 kitten and 2/3 adult with whatever leftover kitten food was around the house. (Approximately) They would eat the first mix, the second mix and then you would switch them to 100% adult food. Doing it this way is fair to them and also economical to you.
But in reality, I just use the recommended age ranges for each formula as rough guidelines. Nano is borderline between adult and senior formula, but if I had a 3-4 year old cat in the household with her, I would try to keep her on adult formula for longer. As it is, maybe in a few months I will buy my first bag of senior food.