Hi fizzy77, I am going through the exact same thing! I began my down regulating drug 5 times a day on Day 21 of my cycle, and was told to take it for 15 days and then they would do my down regulation check (for me that is bloodword & ultrasound). I then have to cut down to three times a day and begin my stimulating injections. They will know if you are ready to begin stimulation if you are "Down regulated" at your check-up (aka: if you uterus lining is thin and your estrogen is low)
Now, I have had an issue because my DR check was last Thursday, but they had expected me to have a period 7-14 days after taking my DR drug. Unfortunately, I didn't, so they want to see me tomorrow to repeat the same test. I STILL haven't began my period, so I don't know if they will have me wait another week, or if they will go ahead and begin stimulation anyway. They just usually want to see low estrogen and a thin uterine lining before giving you injections.
Hope this helps!!