Hi!
The medication should be stimulating the follicles to continue to grow. The ovidrel will mimic the LH surge (which is the surge that is detected on ovulation predictors) and should cause the follicles to mature and rupture. And, you are correct, that should take about 36 hours and the IUI should be timed around that.
There are cases in which ovulation does occur prematurely. It is unfortunate when it does happen because the follicles are not mature and don't fertilize well. If it does happen it will be noticeable on your next scan, and your meds may need to be adjusted for your next cycle. I hope all goes well and you have success!
Good luck!
Dr B
How are you doing? Did you trigger...get IUI? I have to review to catch up. ;o)