It depends upon cell-type and the source of the IFN-gamma. In general, 10 ng/ml of recombinant IFN-gamma can appreciably induce certain MHC genes within 6-12 h. However, the time needs to be optimized with each batch of the IFN-gamma.
I agree with Dr. Choubey that it varies on cell type. Most of our experience is with human culture systems where we have generally used 100 Units/mL or more for periods between 24-48 hours. If possible try to find out the activity of the cytokine that you are using since it varies greatly from manufacturer to manufacturer which makes it difficult to interpret ng/ mL as a unit of measurement.