Search the literature for the IC50 value of the particular drug for this cell line and try starting with this concentration. Your target concentration should be around this value.

I agree with Erkin Aydin. If you want to be more through, you could do an LC50 test on identical cell populations using different drug concentrations. Typically an exponential range of values is used in 96-well plates over a 24, 48 or 78 hour period. Then identify the dosages of interest and test those with your cell. There are several issues with using these values for drugs. The toxicity from drugs is time dependent. That is, higher incubation times increases the cytotoxicity. Hence your LC50 test should be for at least as long as you intend to carry out your experiment.

This might be a good reference

http://ar.iiarjournals.org/content/27/1A/35.full.pdf

Thank you so much Erkin and Smitha