The statute of limitations in law generally says that you have two years to file a civil lawsuit against your employer. If you are looking to make a complaint through the human rights tribunal then that timeline may be shortened to one year (unless there are extenuating circumstances), and if your complaint involves a union then there may be other shorter timelines as well.
However, there is one major hiccup to all of this. If you sign off on your exit package, you have almost certainly signed a release stating that you promise not to take any legal action against your former employer. That is effectively the trade-off for you receiving any extra money. In fact, the release usually says that if you were to file a claim after signing, they could use that release as what we call in law an ‘estoppel,’ or a direct blockade against your legal action moving forward. This is why it is always good to seek legal advice before signing off.