When is the right time?

This guy here, he's been a part of our family for over 18 years. He was our first and only pet. He has slept with us, woke us throughout the night, killed rodents and birds, eaten frogs, been in fights, licked his sister's head until it was raw, snuggled on my pregnant belly, destroyed more bunny nests than I care to admit, brought half dead animals into the house.  

He's always been a big, beefy, hearty, muscular kitty. A good 20lbs for most of his kitty life. 

The past two (maybe even three) years haven't been that great for this boy. He's faced numerous health issues. He frequents the VET every 4-6 weeks for antibiotic and steroid shots. He's even required several enemas due to digestive issues from all the steroids he is on.

He's down to less than 12 lbs now. When I pet him all I feel is his skeleton. He falls down a lot. He eats very little. He won't drink out of a bowl anymore, only out of the toilet. 

He just doesn't seem happy to me. He is old. He is in pain all the time.

For me, it's time to let him go. Time to let him finally rest. For T it's not. We just don't agree on this point. We have said goodbye to him before and T took him in and had him shot up and cleaned out yet again. 

The past few days he has been in obvious pain. Pain to the point that he vomits. He falls over. He spends most of his time laying on the bathroom floor. 

T is bringing him into the VET in an hour. He'll ok more treatment and we'll be back here again in at her few weeks. 

At what point is it time? Here we have this right to make a decision on Quality of life for our pets and yet we (he) continue to let them suffer. Why? Because we will be sad to let them go? Because its too hard to make the decision? 

All I know is I am ready to let him go. I love him and have given him a long and very full life. But I don't think he is living anymore. 

I just wish T could get to this decision too.