We got this question today, so i thought I’d post an answer in case it helps anyone out!
I wish I had a concrete answer for you. It’s so hard to know when the time is right—definitely one of the biggest challenges of having a dog. 😞 I guess it’s mostly a matter of noticing when the scales have tipped to the point that you think your dog is unhappy and/or uncomfortable. With our pet dog Chloe years ago (she was a 120-lb Ridgeback who lived to be 12-1/2), her back legs started giving out. We bought a big soft harness to put around her midsection so we could help hoist her up onto her feet, but eventually she was falling down stairs, into her poop, etc. I don’t remember the exact turning point, but there was definitely a time when it felt like she was struggling a lot and so were we, and it just felt like she was probably ready to rest. We contacted an amazing service that came to our home—I would highly recommend that, if it’s possible. I asked the woman who did it if we had waited too long or should have waited longer, and she just assured me that we had made the right choice. No one knows your dog better than you do, and you have his absolute best intentions at heart. You might always second guess yourself, but if you trust your gut and your heart and think about how much your dog is enjoying himself anymore, that should help you decide. I’m so sorry you’re having to go through this. It’s heartbreaking.