It depends which stage you’re at with your dosing. One pen contains 18mg of the medication in 3ml of solution. When you first start the treatment, you’ll gradually increase your dose, starting at 0.6mg per day for your first week, and rising by 0.6mg per week until you reach the maintenance dose of 3mg per day at week 5.
So if you’re on the standard 3mg dose because you’ve been taking the medication for more than five weeks, one pen will last you for six days.
If you’re beginning treatment at 0.6mg, and then increasing at the recommended rate of 0.6mg per week, your first pen will last you for 17 days. Carrying on from this, your second pen should last you for eight days, then each pen after that should last six days.