The length of time that your Victoza pen will last you depends on your dosage with the Victoza pen.
Depending on your dosage the Victoza pen should last between 30 days to 10 days.
Each Victoza pen has 30 doses of 0.6 mg, 15 doses of 1.2mg and 10 doses of 1.8mg
So it can depend on how much of the Victoza that you're required to take according to your doctor.
Your doctor should prescribe you enough Victoza pens to deliver the required amount of doses of the Victoza so that you do not run out.