When should one really think of doing a PhD?

I am currently doing my Masters. Before beginning with my masters I had plans of starting with a PhD after I complete my MS. But I have been looking into how PhD students work, how much time they devote per day etc. Seeing all this there has been a decline in my interest in going for a PhD or in other words I always have a gut feeling in myself that probably I should not start with PhD any time sooner. I feel that I might not be able to complete my PhD as I actually don't know what I'll do in it!!

So, my question how does one prepare himself mentally for going for a PhD in future?