Generally it depends on the effect of the drug and the model system you use (e.g. do you use organotypic cultures or an in vivo system). I would suggest to define certain readout parameters (e.g. cell proliferation, rate of reepithelialization) depending on the expected drug effect. If information or a starting concentration (from a paper) for the drug is available I would start with pilot experiments and drug concentrations around the starting concentration and then continue the experiments with increased concentrations.

You should be aware that some drugs might have adverse effects if you apply them in high concentrations.