Well, I would say that you souldn't learn piano by ABRSM by yourself. But yes I would say that 8 years could be a fair approximation of time, especially when you start playing as an adult and most probably catch up on things faster.
I started palying the piano as a 6-year-old and took grade 1 at 9 (which was way too easy to me) but at grade 5, I think the syllabus had caught up with me and it became difficult. When I stopped playing I was working on my grade 8 exam at the age of 16, although I never did the exam. My feeling is that the gap between grade 7 and grade 8 is bigger than grade 6 and grade 7. So grade 8 might take a longer time than a year to accomplish.
The ABRSM has also a minimum standard for music theory. You must have accomplished grade 5 in either music theory or in practical exams (playing an instrument) in order to be allowed to contiue to the higher grades. In other words: If you want to do grade 6 in piano you must have done grade 5 in music theory. THAT could be an obstacle depending on what type of person you are.
What I hold against these exam-systems is that it is very easy to become too exam-orientated = you only play those three exam pieces for most of the time of year. Then you don't have the time to go through other pieces that into depth that I would have wanted. You get so busy working on all those different syllabus-things that you don't have the time to REALLY work on your technique. My pedaling still sucks and my vrists and arms are still way too tense (being poorly taught by my first teacher and not having time to correct it with my last teacher)
From my personal experiences I would advice you not to take those exams that seriously but to also play a lot of other pieces and really ENJOY the music! To me the exams made me feel as music only was bounded by rules and more like a handcraft. Music is not a handcraft, it's an art!
Good luck with your piano-studies!