So this seems to be a question that has been around since day one. However while playing ROI at the very start I think we may have our first vague idea has to just that.
Remember when our ghost mentions the lift thing up to the mountain hasn't been used in over 400 years? So does this mean that much time has passed assuming it was last used around the collapse?
I don't think so as we find a pamphlet talking about bringing the kids or something along those lines.
I do think it has definitely been centuries though given how Lord Saladin talks.
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Hundreds I'd say about 2-300 maybe less doubtful it's higher otherwise we'd have been all dead ages ago