There is no time limit, however, the older the incident gets, the less likely it is that your girlfriend has a reasonable fear of imminent harm. In order to have her petition for injunction granted, she must prove that she has a reasonable harm of imminent danger. If she moves back in with you and files for an injunction based on this altercation the next time she gets mad at you, it isn't likely that the court will believe she has a reasonable fear.
You should contact an attorney of your choosing immediately if you are served with a temporary injunction. Although the injunction process is designed to be friendly to pro se litigants, injunctions are very serious and the advice of a professional is well worth it.
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The statute limiting the time to file charges for a first degree misdemeanor (simple battery) is two years. If it qualifies as felony battery the time limit is 3 years.