How much faraday of current is needed to produce 1mol of O2 from 1 mole of H2O?

Dear student,
According to Faraday's law of electrolysis the amount of substance liberated at any electrode is directly proportional to the amount of electricity passed through it. 
${20}^{2-}\to 0_2+4e^{-}$
since there is a change of 4 electrons.
Hence, 4 Faradays will be needed to liberate 1 mole of O₂ from H₂O.
Regards.