This is a controversial subject. You will need to discuss these issues with your doctor up front before starting the cycle.
Many doctors are reluctant to cancel unless all indications are terrible.
Because of the emotional and financial cost of any IVF cycle you may want to cancel a cycle before retrieval but be aware that you may meet with resistance from your doctor. You may still choose to continue with a cycle that doesn't look promising but be sure it is an informed decision. You also need to remember that the donor is also a patient of your doctor and your doctor is obliged ethically and legally to provide the best care for her as well as for you. There may be a problem if the estradiol level stays low or plateaus for more than 3-4 days and there is definitely a problem if the estradiol level drops. Most of the follicles should grow in size at about the same rate.