On average, on disposable Lantus and Basaglar pens, I average about 10 units in the reserve. The big bonus is that you also get to draw out those small partial doses that would have been leftover from your full dosage needs. This, plus the reserve, gives me an average of one extra day in a box of 5 pens. With the basaglar pens, you can also take a dose from a full pen, then turn it back enough to allow you to inject your salvage syringe back into the delivery chain.
I even do the same process with my “refillable” cartridges.
With the price of insulin these days, the only unused insulin around my house is the amount needed for those pesky priming shots!