How do I know when to file a lawsuit?

The short answer is, "After you consult with an attorney."  There are many issues to address before you decide to sue.  First, have you tried negotiating with the other party?  Has the statute of limitations run?  For personal injury, the statute of limitations is three years.  For slander and libel, it is one year.  Do you know where the wrongdoer lives or conducts business, and does the potential defendant have any assets or insurance coverage that would make your case worth litigating?  What evidence do you have to prove your case?  Do you have any witnesses?  After all, it is the plaintiff's burden to prove every element in civil litigation.