There are many different kinds of cysts and tumors of the ovary. In an adolescent, there are certain tumors of the ovary that are more common than at older ages. Given the size, it is unlikely to be a functional cyst which is a cyst related to the menstrual cycle. (please see my 8/17/06 answer to the 8/13/06 question on "complex cyst and free pelvic fluid" for a discussion about functional cysts.)
a 20 by 8 cm cyst could be a benign growth or a malignant growth. The most common benign growth is a dermoid cyst (also known as "mature teratoma") Other benign growths include serous cysts, mucinous cysts, and endometriomas. Cysts can also come from another pelvic structure such as the bladder, the fallopian tubes , or the lymph node region of the pelvis.
All these things are non urgent and waiting 3-4 weeks is just fine. It usually takes years for these tumors to grow to 20 cm.
An infection in the fallopian tube such as an abscess can occur. Most people are really sick when a tubo-ovarian abscess occurs with fever, pain, elevated white blood cells count. I would assume that your daughter does not have the clinical signs to suggest this diagnosis.
There are a few extremely rare malignant growths that can be seen in teenagers. The general term is "germ cell " tumors which means that the tumors come from the egg (also called oocyte). Malignant germ cell tumors are rapidly growing tumors that develop over a few months. If there is any suspicion of a malignant germ cell tumor, a person needs surgery immediately. There are certain blood tests that help understand if this diagnosis is a possibility.
They include: bHCG (beta subunit of human chorionic gonadotropin)
afp (alpha feto protein), and LDH (lactodehydrogenase)
You should ask her doctor about the results of these blood tests.
There are other malignant tumors that are not fast growing at all. These include borderline ovarian tumors. The CA 125 is the appropriate blood test to get for this. And granulosa cell tumors or juvenile granulosa cell tumors. Estrogen and testosterone levels are appropriate to get to evaluate this.
Overall, it could be that your daughter has the usual adolescent immature ovary - pituitary-hypothalamic axis that leads to irregular cycles plus has a benign tumor on her ovary. That would be the most common situation. However, she could have a tumor that is secreting hormones which is then causing irregular cycles.
Luckily just about all the things that adolescents get on their ovaries are easily taken care of and even the malignant tumors have a good prognosis.