When is the right "time"?

This is the last time Samson and I spent time outdoors together. September 2015. He doesn't want to hang out anymore. He seems to be more comfortable indoors, sleeping.

I found Samson on December 9, 2003. It was bitter cold and snowing, with the nights dropping below zero. I had been to my property in the country where my aunt told me she'd seen two pups dead in a ditch down the road a few days prior. While driving by, I noticed one licking his paw. I immediately stopped my car and called a fellow I knew to ask him to come get the pup and put him in my Blazer. He did. Samson (as I later named him because I'd asked the Lord to restore his strength and fur) sat in the front seat just looking at me. He was so cold, his body was numb and he couldn't feel his broken hips and deep wounds. He was in the veterinary hospital for three days. He's been with me practically 24/7 since.

Samson has had multiple surgeries since for various tumors that pop up. He has a parathyroid gland tumor, diagnosed a year and a half ago. He's lost much weight, but is not emaciated. He is weak; stumbles and falls; appears to not see or hear well. He stands in one place often ... seemingly confused as to what exactly he wants to do. I have to really coax him to eat. He drinks two or three times a day but nothing like he once did. He sleeps all but a total of maybe an hour or two out of 24. I took him outside today to enjoy some sun. He always loved that. Twenty minutes seemed to exhaust him. He wanted in, made his way to the couch and is now sleeping.

I can't imagine how he will endure the winter. He gets dry heaves often; needs to eat grass. There really is none in the winter without digging through the snow. Since I use a wheelchair, I cannot get into the deep snow to assist him. His potty breaks are difficult because he doesn't like going out into the grass. He's never liked wet grass.

How do I decide when it is time to let him go? When is it time to cross that Rainbow Bridge? I don't want Samson to suffer. But I certainly don't want to shorten his life because I made the wrong decision.

Have you lost a furry friend this way? How did you make the decision? Do you have any regrets or wish you'd done something differently?

I know this is a difficult topic and if you choose to respond, I will appreciate it with all my heart.

Blessings ...