When is a good time (during graduate studies) to start thinking about writing the thesis? A significant number of colleagues had their thesis defense in the past year and talking to them got me thinking about my own dissertation to come in about 2 years (I am approximately half-way through my graduate studies).

Based on my experience, the norm appears to be so that the focus of a PhD student shifts from initial getting-used-to-things, to getting-papers-out, do your coursework/teaching (if applicable), getting-papers-out (repeat)... until you are a couple of months away from the dissertation date, and you panic your way into writing your thesis. The panic then leads to stress and errors, which I witness with many colleagues. 

Seeing as my projects are particularly detached from one-another, it would perhaps be good to start thinking about some structure, and perhaps even start jotting down some text; not long paragraphs but at least some bullet points on the points I want to mention, statements I want to make in my thesis. 

Seeing that I am only half-way in, is this a reasonable approach? ... or am I stressing way too early? Does one need to leave the thesis writing to the end when the stress is maxed-out, or are there smarter ways to handle the writing? I imagine one drawback of starting now, is that I do not yet have the "bigger picture" but yet again, since the projects are not built up on one another, I don't know if it's a real issue in my case. 

Any perspective on the matter is welcome, of course.
I think it is a good idea to start writing down parts of your thesis early on. You need to consider possible structures for the thesis as well so that you know what you might need to write.

Starting early will, however, not remove the need for significant efforts at the end because the thesis evolves and it is only when you have your last papers/experiments/equivalent done when you really know how the text will have to look. This means you should write down text early on with the clear understanding that this text will likely undergo lots of revisions. While this may seem like a waste of time, I have found it useful to have the thoughts down on paper, it also provides a sense of fulfillment, "that you are on your way". In some cases I have had to scrap the texts altogether and start over but I see this as part of the iterative process of scientific work. 

By going through the iterations your text will improve and the last efforts will be ever closer to a final product than were you to start from scratch at the end. I went through this myself and I have seen countless students do the same. Being prepared and realising that much awaits ahead is just a realisation that must be clear. Going blindly into the final stages without having a grasp of what is needed is what causes extra and unnecessary stress.