I just took a quiz, and I think I did very poorly. It says I got 0.52/5.00 -- granted my spelling is poor, I'm not finished with the course, and I know I hit enter too soon or bumped the touchpad on my laptop on some questions. Next time I will turn the touchpad off.
I noticed that some of the questions were things I haven't seen yet in the course -- though as the quiz went on, it seemed to figure out where I was and give easier questions. When is the best time to take the quiz? What kind of results have you gotten?
Personally I think it's best to use it as it's designed - a progress quiz. Doing it regularly, you will see how you progress. I meant to do it at each checkpoint but I kept forgetting. It's okay to get a low score, it isn't pass/fail, it's to assess how you are progressing.
It will test you over the whole tree, but it will also adapt (somewhat) to your success; the more you get right, the further down the tree it will go.
Three months ago I scored 1.88 and three weeks ago I scored 2.33 in Spanish. I may try again next week.
I don't know it there is a best time to take the quiz, you could say it's best to wait until you finish half the tree or even the whole tree since it tests you on the whole thing but it's fun to watch your score rise and lingots come pretty easy so there is no harm in taking it as soon as you can afford to.
The first time I took the quiz I got a measly 0.38, and that was only two weeks ago. When I have enough lingots I plan to take the quiz again to see what progress I've made (if any). I love working through the Russian tree and think if I can see some signs of improvement, then it will be a huge encouragement and add to my motivation. Personally, I would prefer to see my quiz results gradually improving the more I practice, rather than focusing on gaining a large streak count.
The difficulty of the quiz doesn't depend on how far you are in the tree – you get quizzed on all the material in the course. Also, as you make mistakes, the quiz starts giving you easier questions, to better gauge your level – if you failed a lower-intermediate question, you'll fail an advanced question, so it's more informative to ask you something simpler. It works the other way around, too.