A single Staedtler, when I was wise enough to store it horizontally, lasted me 3 months, during which I had been able to go through Lesson 1 and roughly half of the 250 Box Challenge, doing warm up exercises every day.

But you must be very careful: do not press hard on the nib, keep the pen almost vertical when you draw and, at least for the Staedtler, store it horizontally.