i finished grade 8 last year and im doing my certificate for completion this year.... for me its taken a little under a year to do each grade. It really depends on how much you practice or how good you teacher is. If you want to self teach and do a good job of it i suggest investing in a music syllabus that is recognized across your contry. For me it was the AMEB. Ive played for almost 8 years and im still having fun =).
Best of luck!