I am not an expert on CR, but I am with vistana, three days is long enough.
I have gone for six days, and while wearing a crotch rope(a crotch rope keeps me in a constant state of arousal). That in itself for me was quite an accomplishment. It was a punishment and I did get into a proper mindset for it, but would never want to do it again.

I would start off with three days, but that is my own biased opinion, you can eventually work on longer time periods, depending on how long a person can go without being completely frustrated by it.

And damn I wish I could learn how to cum on demand with just one word, I find that to be really difficult to manage.