You must file a defective product liability lawsuit within a certain amount of time. The legal term for this time limit is the "statute of limitations." Every state has a statute of limitations that applies specifically to defective product liability claims, but the actual amount of time you have to file your lawsuit varies from one state to the next. If you fail to file your lawsuit before the statute of limitations expires, the judge will almost certainly throw out your case, no matter how good your claim may be.
The moment when the time limit begins to run also depends on the law of your state. In some states, the clocks starts ticking on the date of the injury, but many states don't start the clock running until the injury is discovered "or should have been discovered." This can make all the difference in cases in which you don't realize you've been injured until months or even years later (such as cases involving drugs, for example). To learn more, read Nolo's article Time Limits for Filing a Defective Product Liability Claim.