This will depend on how aggressive you drive your car and how often your are triggering injection .
Meth doesn't spray all the time so if you drive normal / you will never have to refill.
Also keep in mind that Meth nozzles come in sizes that determine how much volume is actually used.
I...
I am in process of installing a meth kit : here is what i am using to cleanly tap for a boost reference:
p3 boost tap for BMW b58
www.p3cars.com/bmw/p3-boost-tap-bmw-b48-b58/