As the disease progresses, you might see some stumbling and some pain. You should talk with the vet about medications to control the pain BEFORE you need them.
As far as life and longevity go, that is a personal choice that you must make.
I counsel people on this topic all the time, though, and I can tell you what I would tell them.
You should choose three things that your pet loves to do. It might be riding in the car, eating cheeseburgers, going for walks, chasing a ball. As he gets older or, maybe, sicker, he will cease being able to do these things that he loves. When one thing is gone, like chasing the ball, it's ok because he might take up a new activity like sleeping in front of the fireplace. However, when you can no longer find even one thing that he loves to do, it might be time to reevaluate the situation.
As far as guilty feelings go, you should not feel guilty if you decide that your pet is no longer living a happy, content, productive life and you help him to go on to a better place. You have to remember all the good times that you have had with him. We are lucky in that we are allowed to help our pets to move on. Sometimes I wish that we could do the same for people.
Let me know if you have more questions.
I am very sorry about what you are doing through and truly feel for you.