Oh Amy, that is a long time! I got my BFP last Saturday. From what I can work out, I will be 6 weeks at some point next week! 
We started TTC in November 2014, but had to go through a lot of tests last year as I never had a real AF since stopping the pill. I was put on clomid in June last year, but 50mg and 100mg didn't work. 150mg did make me ovulate the first time, but not the second and it was the third round (when I had given up hope and wasn't expecting it) that led to my BFP!
I only had cd3 tests at the beginning of my journey, when I first went to the GP about my lack of AF. As far as I understand it, with the cd3 tests, they are checking two hormones (someone please correct me if I'm wrong!), and these hormones are the FSH and the LH. I think that you want these to be low and they also need to be close together. If one is a lot higher than the other, this can indicate problems. (Mine were 4.8 and 4.9).
CD21 tests check for progesterone levels, which is the hormone that is released after ovulation. They are looking for this to be over 30, which would indicate ovulation around cd14 on a 28 day cycle. (It rises rapidly after ovulation). You should be asked to make appointments for cd21 tests after every clomid cycle, to check that it is working. The first time I took 150mg clomid, my progesterone on cd21 was 33, so it had clearly worked. Every other time on 50mg and 100mg, it was only 1 or 2. The cycle I got my BFP, I went for cd21 tests and my progesterone was only 14, so they told me that it 'probably hadn't worked' and had made arrangements for me to have a follow up appointment to discuss 'next steps.' A few weeks later, I found out I was pregnant. This means that I ovulated late in my cycle (around cd18/cd19) and therefore the prog levels hadn't reached even half of what they should have by cd21, so don't worry if your levels aren't quite what they should be. Good luck! x x