At what point in your PhD journey have you felt that you really REALLY do have substantial knowledge of your area?

Usually about five minutes before I find something that makes me feel like I am back at square one!!!!!

In all seriousness, this is true--but I am not discouraged by it. I will be going along, thinking I am FINALLY well and truly on the topic, and then discover more material, another angle, something I failed to account for...and its back to the drawing board. What I have decided though is not that this means you do not know your topic, its just that you are going back for an enriched meaning. Its like a spiral, you keep covering the same ground, only looking at it differently.

I would have told you I knew my topic the day I started. I would say that today--but how I knew, how deeply and abstractly I knew it--the dimensions with which I can come at my topic and consider it--its relatedness to other areas...those are different.

In other words, there is always more to know. Its infinite. In my last supervision session, my supervisor and I were brainstorming about different linked fields, very enthusiastically, until we both realised there were limits--and that though interesting, many of the avenues we had mentioned would have to wait--there was no way to get them on board the current PhD! That said, one feature of good research writing is that which discusses the limitations of the research and gives ideas for future research.

It took me nearly three weeks.
Only joking! It's hard to say, I would say that I now (after 6 months) understand the area I am working in fairly well - but it's only a tiny area and anything outside that is just nonsense to me. I know a postdoc who feels similarly!
This is in mathematical sciences, though, and seems to be very different to what most other people on here are doing. There was no need to read extensively, design or plan methodologies etc. it was pretty much starting to produce results in the first few weeks.

Hey big_mac: my PhD is in statistics so not too different a field. I am working on methodology, so agree that it's not quite a normal PhD is terms of Data collection, Methods, Results etc. etc.

When I started my PhD, I was hugely unaware of what I did not know--the unknown unknowns! So I could be blissfully ignorant....that there was a lot more I needed to learn! Thankfully it reveals itself over time and in small bites...otherwise it would be too overwhelming and I never would have taken on the PhD! It would have been too daunting. So...its OK to not know everything, its probably even better that you do not know that you do not know a lot on your topic, and an awareness of how much more there is to learn is a sure sign of progression!