Unfortunately, the law in every state, including California, doesn't provide for an unlimited amount of time a person can wait and still be able to file any type of claim. This includes California and it definitely includes claims for wrongful termination of employment. Generally, if the claim arises from discrimination, harassment or retaliation related to a race, gender, age, disability or medical leave issues, as well as other related claims, you must "exhaust your administrative remedies" by filing a claim with either the California Department of Fair Employment and Housing, the Equal Employment Opportunity Commission, or both within ONE year from the date of the conduct for which you are complaining (this can be the termination itself or, possible, earlier "adverse employment actions" like demotions, write ups, etc.). Once they issue a "right to sue" notification, this provides for one year from the issuance to file a lawsuit in court.
Other "common law" claims like breach of contract, defamation, or wrongful termination in violation of public policy may provide for a longer time limit (between 2 and 4 years depending upon the type of claim).
HOWEVER, this is only ONE part of the analysis. The problem that I see, many times, is that, although someone has a right to file a suit within a year, or sometimes more, from the date they were fired, they have missed an opportunity to "preserve the evidence" and conduct the proper investigation if they don't act MUCH sooner. Many times, I have consulted with people WHILE THEY ARE STILL EMPLOYED and advised them to document certain things that become crucial evidence later on in the case.
The bottom line is that it is never TOO soon to consult an attorney and anytime someone is having problems in the workplace that they feel are illegal, they should IMMEDIATELY consult an experienced employment attorney to determine what should be done to either remedy the problem, preserve the evidence that will make their case much stronger later, and to file all the appropriate claims on time so that they don't lose their rights to seek redress.