The most reliable way to find out is to perform a dose response/kill curve for your cell type. 

Hello Kevin, I already started, however, after three days, I haven't observed any cell death yet. Ranging from 0.1 to 1 mg/ml.

Three days and no visible cell death @ 1mg/ml?  Thats unusual, is your G418 expired ? :), I usally see lots of cell death after 2 days at that dose (other cell line).  I change my medium with fresh G418 every 2 days, and at day 7 all cell are death with that dose. 

see if this paper can help, they use 300-600 ug/ml for Y79: http://cancerres.aacrjournals.org/content/52/22/6297.full.pdf

Thank you very much Kevin