current of a galvanic cell
From Wikipedia, the ampere:


  The SI unit of charge, the coulomb, "is the quantity of electricity carried in 1 second by a current of 1 ampere". Conversely, a current of one ampere is one coulomb of charge going past a given point per second:


$$1~ A=1~C/s$$ 

and the coulomb:


  It is equivalent to the charge of approximately \$ 6.242×10^{18} (1.036  ×10^{−5} mol) \$ protons, and −1 C is equivalent to the charge of approximately \$ 6.242×10^{18} \$ electrons.


So, from the above:

$$ 1~A = -6.242 \cdot 10^{18} ~ electrons~ per ~ second.$$

To figure out the current you'll need to know the number of electrons produced per second.