The insulin prefilled pen or cartridge typically contain about 300 units of insulin (In some cases it may have more depending on the manufacturer). Hence, the duration it will last will depend on the ‘dose’ that you are using daily.
For example, if the dose of your insulin is 20 units daily, the pen/cartridge would last you for less than 20 days. We say less than 20 days because you need to prime your insulin device with two units every time you use it.
An opened insulin prefilled pen/cartridge typically lasts for 28-30 days when stored properly and then needs to be discarded.
Unused insulin cartridge and pen, when stored in the refrigerator as per the manufacturer’s recommendation (typically around 2-5 degree Celsius temperature), can last till the expiry date mentioned by the manufacturer.