Transfections may be repeated to maintain knockdown. You can see the attached file for details.

Thanks dear 

Yes but make sure that there is no toxicity associated with transfection. You are also stressing cells with repeated transfection which may affect cell viabilty and gene expression profiles. You may have to conduct some type of cell viability assay to test cell viablity after each transfection (of course on a set of control group as a surrogate). A better strategy is to use vector-based systems but they are costly.

Thanks Attila