The statute of limitations refers to the period after a crime when the District Attorney can file criminal charges. The statue of limitations depends on the offense: for most misdemeanors, it is one year; for felonies, it can be three years or more. For some offenses, such as murder, there is no statute of limitations.
In many places, the police won't promise you anything for being an informant (or snitch, as they're known on the street), except that they will tell the judge you were cooperative. Very often, they will want your information before telling you what they will give you in return.
I have seen cases where informants provided valuable information to law enforcement, but only received a slight reduction in their sentence in return. They then go off to prison with a "rat jacket," meaning other prisoners know they were informants and consider them "no good," along with child molesters and other unpopular groups.
Your only rights are the ones that you received in your agreement to become an informant... and you may have a hard time proving what they promised you.
The time frame for a "rearrest" doesn't necessarily come into play here - as long as the District Attorney filed charges within the statute of limitations (in general, one year on a misdemeanor and 3 years on most felonies), then the case can go forward.
Your issue is whether or not you are getting the deal you thought you were supposed to be getting. The police have no legal authority to offer you any deal in court. That can only be through the DA's office. If you provided information, there are ways that can play out in your favor, but that is something for you and your attorney to discuss face to face and NOT on a public forum such as this.