There is no set answer to that question.
The DSM-IV-TR (2000) says that to be diagnosed with OCD, a person must have obsessions or compulsions that last more than an hour a day, that are significantly distressing, or that significantly interfere with daily functioning.
Therefore, even if a person spent less than an hour obsessing each day, the obsessions could still be so distressing that they would meet the diagnostic criteria. Or, a person may obsess for several hours a day.
There really is, however, no typical amount of obsessing. Each person is unique in the way he or she is affected by his or her obsessions.
Further, there may be some days where a person obsesses very little, but then the person may spend entire days obsessing at other times.