The pen holds 300 IU... so how much do you take a day add in 2 IU for each injection (priming)?
divide 300 by the daily insulin used will tell you how many days a pen will last
how many units per day are you shooting? take that number and divide into the capacity of the pen and you have your answer.
The above comments are correct for the overall estimate if you take the same doses. My doctor has me on a sliding dose scale which depends on my pre-meal glucose readings.
Depends on how you use your insullin. My doctor gave me an insulin to carbohydrate ratio to use. I use 1 unit of insulin to every 12 grams of carbs I eat. The less carbs I eat, the less insulin I use. I have a very physical job for four hours a day and I don't even need my Humalog when I eat lunch. My pen generally lasts me the whole month. My Humalog is only good for 28 - 30 days. As far as I know, once your insulin pen is opened, that's how long most of them last no matter how much insulin you use.