As this thread shows, there is no hard and fast rule here, people find what works for them, and that is the status they use.
I would never have dreamed of referring to myself as 'step-father' to our two girls when we met - they were six and four, and there was no specific time when things changed - they just evolve into what everyone is comfortable with.
Now they are thirty-seven and thirty-five and as they always have, the call me Andy, and they refer to me as their dad in conversation. Our youngest daughter calls me Dad, and the three of them call me their dad in conversation between them. Even now, i get a flush of pride when i hear them refer to me as their Dad, which is what I am. Their 'father' did the easy bit, the 'dad' part is the hardest. and most rewarding, and I am so glad that's the role I got.