There are 5 levels of Chronic Kidney Disease that doctors use to determine how advanced your kidney disease is. Level 1 and 2 are mild, 3 is medium, 4 is advanced, and 5 is end stage renal disease (needing a transplant). They are determined by your GFR (how well your kidneys are functioning - a GFR of 50 would roughly equate to your kidney's working at about 54% for example - so GFR roughly equals percentage of kidney working)
How long each stage lasts will depend on the cause of the dysfunction. I actually went from stage 3 to stage 2 with diet changes so it really depends. I was told I could stay at stage 3 for a lifetime, or that I may experience a more rapid change. That they just didn't know. That they would monitor it every six months to see how things went.
Good luck. Hope with diet things hold out.
just a hint, please: what stages are you talking about?