Making a report to the police is sometimes referred to as "pressing charges" or "filing charges." But the way the system generally works is this:
1. A crime is reported to the police.
2. The police investigate the alleged crime.
3. The police can then choose to do nothing, or they can prepare a report on their investigation to send to the District Attorney's Office.
4. The District Attorney's Office reviews the police report.
5. The District Attorney's Office can the refuse to prosecute, or they can decide to formally file a charge in court.
So, reporting a crime to the police is a necessary step in the process of a charge being filed. But the report does not guarantee that a formal charge will be filed - either the police or the prosecutor could decide that no crime was committed or, even if a crime was committed, it might not be worth pursuing for some reason.
Also, it should be noted that once a report is made to the police, it is not necessarily in the victim/reporting party's power to terminate a prosecution. The police and prosecutor can go forward with prosecution even if the victim/reporting party wants the whole thing dropped.
The police and prosecutor have up to 2 years to formally file a charge. But, if a report is made to the police long after the alleged assault, that will obviously tend to cast doubt upon the truth of the allegations. But I can't tell you how long is too long - that would be up to the police and/or prosecutor.