Statute of limitations issues in medical malpractice cases can be tricky. In general for adults, the statute of limitations is two years from the date of the negligence. There are times, however, when the patient doesn't know that negligence has occurred until later. This is sometimes the case in cases where a misdiagnosis is alleged, because the patient didn't realize that either the diagnosis or the treatment wasn't right until later. For these cases, Illinois uses a so-called "discovery rule" which gives a patient 2 years from the date that a "reasonable person" would have suspected that the diagnosis or treatment was negligent. Notice that the rule is what a "reasonable person" would have done or suspected, not what the patient actually thought. This means that if a reasonable person would have begun to suspect that the treatment was wrong long before the patient actually did, the statute can expire. There are a number of additional rules which might also apply, depending on your facts.
In the end, the safest thing for you or your finance to do is to consult with an experienced medical negligence lawyer, lay out all the facts, and get an informed opinion about when the statute will run. The best advice that I can give you is not to wait-- cases almost never get better as they older. There's nothing to be gained by waiting and you risk letting the statute expire on a meritorious claim.
While the general statute of limitations for a medical malpractice case is two years, an answer specific to your facts may be different. For some facilities, the statute of limitations is shorter, in some fact patterns, it may be longer.
If you believe your fiancee was injured as a result of medical malpractice, it is imperative you contact an attorney as soon as possible. Only by analyzing the specific facts of your fiancee's case would they know whether or not there is a viable case.