how to know when my dog should

Question
--------thank you , i looked at the web sites you suggested & learned a lot.up to now i thought that my dog was just getting old like others but when i read the symtoms of CDS i knew that she is suffering an actual disease & it has been coming on for 2 years. my husband & i will take her to the vet this week as she is too unhappy & far too gone for treatment.What i want to know is ---is CDS hereditary, she has PRA which is & are  the 2 conditions  related in some way  ?-----------------
Followup To
Question -
my lhasa apso bitch is 16 1/2 and has been blind  4/5 years. over the last year she has lost interest in us [does not want to sit with us] looks depressed only likes her food ! she will walk though not keen on hills.now she finds it impossible to find her way out into the garden so wets in the house, she seems to know when she wants to "go" & wanders about, of course we then put her out & remarkably she finds her own way back in. i dont think she has lost her intelligence that makes our decision so hard. she is very thin but does not seem ill except that she has an intractable infection in 1 ear only.my last dog was 14 when we had him put down because he was very ill. in a way that decision was easy, but this time i am finding it difficult. but things are difficult for us as my husband has much to do as i have a broken leg & cannot help much with her & the mess.last night she had diarrhoea. how bad  should i consider letting this situation go on?most of the time my dog sleeps.
Answer -
I would recommend trying ANIPRYL, www.lbah.com/anipryl.htm  which is a medication for dogs with senility (cognitive dysfunction) as well as trying to get her to eat B/D to see if that improves her mental capabilities.  www.gentlepetcare.com/BD.htm

As far as euthanasia time, I always tell people it is QUALITY of life and not quantity.  If the bad days outweigh the good and your pet has lost that sparkle in the eye and is feeling sick, then it is time.

Answer
CDS is due to aging and most likely from free radical damage over time and age related degeneration.

I do not think PRA is related, however, if reduced function is leading to reduced oxygenation of tissues it could contribute to degeneration over time.