if a plaintiff decides to not press charges what is the statute of limitations on battery

Your question is a bit unclear -- there is a difference between the statute of limitations in civil court (one person suing another for battery), which is two years, and the possible criminal prosecution by the State against the person accused, which could be one, two, or three years depending on some facts that were not revealed in your question.  And "in self defense" and "on accident" are pretty much contradictory positions.  If the alleged victim is non-cooperative, the State may decide to drop the case, but if it looks like charges are even possible, the best bet for the potential defendant is a consultation -- immediately -- with qualified criminal defense counsel.