re: When do you know it’s time to put a pet down?

Sounds like its time man. 17 for a lab is an exceedingly long life. My family had to put down our 12 year old lab last year when he developed cancer and lost interest in eating.

Your dog can't tell you hes miserable but if he can't get up, he can't control his bowels, what quality of life is left for him? He can't do the things a dog loves anymore. Like I said he can't tell you how much pain hes in and I imagine you don't have endless resources for human like end of care to keep him comfortable until he passes naturally. So its up to you to decide for him when he's had enough.