Depend on indication. For ovulation (when a patient who doesn't release an egg on her own), then assuming there are open tubes and normal sperm, Clomid (clomiphene) restores normal monthly conception rates: about 20%/month for women < 35, 10%/month for women > 35. When used for unexplained infertility, then it is less effective, generally about 5-7%/month depending on whether or not IUI is also used.
Ovulate or Pregnant. While up to 80% will ultimately ovulate with clomiphene, only about half of those will conceive. Before using clomid, a sperm test is needed. Consider an hsg as well. Three months at most before considering other options. Many form large cysts with clomid, so an ultrasound is helpful at the start of each treatment cycle.
Estrogen receptors. Clomiphene is believed to block the estrogen receptor in the brain. The brain in turn produces hormones that stimulate the ovaries to produce eggs.