There are several factors that you have to take into account. Size is one. Cinnamon, or what we call cinnamon, is sold in 2" increments and can be found anywhere from 2" to 12".
Storage is also important. If your pulling it off your spice rack that sits right behind the stove it will be considerably weaker.
Age is another factor.
I put 2 2" sticks into three gallons and it hardly did anything. When I added 2 sticks (fresh) to another batch it was way to strong
Start light and increase if you need to.