Pardon me for trying to be funny but I think as much time as it takes. Myself I don't think you can put exact minutes on building a successful business.
Five minutes a day is not going to produce much results. I spend as much time as I need to do the things that are necessary to do my daily activities and then I move on to advertising. Sometimes it takes 2,3,or 4 hrs but being my own boss I do as much per day as I can.
