Do Write ups stay forever?

So, if you get written up for a problem, does that hang over you for the entire duration of your employment? My boss said that getting written up 3 times is grounds to be fired. I got 3 different answers from 3 different people. One coworker said it's bullshit and a write up will be erased eventually. One coworker said it DOES stay, but it only counts if I get written up 3 times for the same offense. And my boss said that it THEORETICALLY stays forever, but refused to clarify on what 'theoretically' means in this case. What is the truth? What does a write up mean in detail? Am I permanently one step closer to getting fired? Is it only if I get written up for the same thing 2 more times, or is it broadly 'if I get written up for anything 2 more times?'just trying to figure out how much I need to worry.