That's normal.
30mL seems like an awful lot. Even in the IBTanked brochures/instructions that comes with the tanks, they say around 10mL per carto is average.
There are many factors that play a role in how long your cartos last. Carto Resistance, proper priming, how many volts are used, which e-liquid is used, etc, etc.
Anywhere from 6mL-15mL is normal for me, depending on the liquid. It just depends on many factors