Ok. Although a few times I went through all the locations but still couldn't find one of the keys ¯\_(ツ)_/¯What do you mean by time limit ? There is none. The person saying the keys disappear after a set time is wrong, the first time i spent more than 10min to find the last key, once you know every location possible it doesn't take that long. Most people tend to forgot that a key can spawn in the control room where the beds are.
You can find a guide here to help you with the different key locations :You must be registered to see the links
You don't need to know what's written, the screenshots should be enough.
Oh well, I completed it all anyway already.