It's a very personal decision and everyone makes it differently. Heck, even the individual dog you lose will have an impact on whether you want another one or not and/or how long before you decide to go for it again.
Terryallan, I understand your extreme dislike of having to put down your pet. But, one thing to remember, is that no matter what you do, every animal out there will die. I have taken in my pets and the pets of friends, as well as strays. None were easy, but I have to believe that I made their deaths more peaceful than it would have been otherwise. I do not want to die alone and/or in pain - I don't want anyone else or any animal to do so either. If that means I have to go through it again, then so be it. I do it for them - my pain means their release.