You should file a police report as soon as the crime happens, but many people have reasons for not wanting to do this immediately. Maybe your family member committed the crime, or you're afraid of retaliation. Sometimes you might not even be aware that a crime has occurred until long after the fact. But most states and municipalities have time limits and deadlines for when you must file.
TL;DR (Too Long; Didn't Read)
The deadline for filing a police report after a crime varies from state to state and from crime to crime. As a victim, it is in your best interest to report the crime as soon as you can because the sooner you report it, the more accurately police can investigate it.
Statutes of Limitations
All crimes have statutes of limitations that govern how much time can pass before the state begins prosecuting a crime. If charges aren't filed before this time expires, the case is effectively dead. The state can no longer bring charges. The filing of a police report is typically the first step in a prosecution and it may be weeks or months before actual charges are filed after you make the report. If you wait until the statute of limitations is almost up, the prosecutor and investigators won't have time to prepare a case and bring charges.
Although it varies from jurisdiction to jurisdiction, the statute of limitations on petty offenses is typically one year. Petty offenses are those that involve minor violations of the law, such as traffic accidents. Technically, you could wait up to 11 months before filing a report and officers would probably still have time to investigate the case and bring charges.
Misdemeanors can involve jail time if a suspect is convicted. Simple assault and domestic violence are typically charged as misdemeanors. In most jurisdictions, the statute of limitations for filing these charges is two years, but victims will often take some time to decide whether they want to file charges against a friend or family member. Because misdemeanors can require more investigation, a police report should be made at least six months before the statute is up.
Felonies are the most serious crimes and the statute of limitations for these is typically five years. In some states, crimes like murder and rape have no deadline. You could wait several years before filing a felony complaint out of fear or loyalty to the person who committed the crime, and this sometimes happens in cases of child abuse or sexual assault.
Variations In the Law
Statutes of limitations vary from state to state. In some, the law requires that you must report things such as car accidents, and you only have a few days to do so. If you think you have reason to file a police report, talk with a local attorney who can tell you the exact rules where you live. For most crimes, the time limit starts the minute the crime occurs, but with others, such as theft or child abuse, the time limit doesn't start until the crime is discovered.