I think NN/G's model is one of the most comprehensive ones that make sense and cover all the different level. There is a series of articles on that (as someone mentioned in the comments) explaining the 8 stages.
However more specifically, I participated in one of NN/Gs seminars and they explained at length what the different stages mean and what they entail for a company. They even provided a survey for us participants to fill in that would help us figure out at which level our company's UX maturity stands. This is probably closest to what you need and would probably help you to define your organisation's UX maturity (at least by NN/G's standards).
As an example, according to the survey, Facebook was at stage 8 on that scale.
Beyond that, the main gist was that it takes time to go through the 8 stages, years and decades even, but those time limits can be affected by a ux-centric management presence or someone higher up that is willing to make the switch into a more UX friendly structure.
Unfortunately, I do not have the ability to share that seminar material with you and I cannot seem to find the survey anywhere online.
However you can find multiple sources around the net that give a decent indication. I hope this info is helpful somewhat and can give you a better perspective.