My Wife Kidneys Are Not Working Properly They Are Measured

My wife kidneys are not working properly they are measured at 25 we have been told when the kidneys drop to 15 she has to go on dialysis. Can you give us some idea how long it would take to decrease from 25 to 15.. Can this happen all of a sudden or is it a slow process. Her age is 81 and she has been on high blood pressure medication for 14 years.
Thank you.