you don't know me, i'm just a junior member. but i've just recently been thru this w/one of our long time pack members.
it took me about 2mos to make the final decision. probably it was too long a wait, but i just couldn't do it sooner. finally the day came it was just a day like any other. but i looked at Teddy sleeping and thought, that's it. it's just too much pain for too long, and it has to end today while the pain is back a bit, on a good day. so Teddy's last day was calm and sunny and happy and i can remember him that way, and he can leave this world that way too.
we had talked about it extensively between the family members. i didn't separate the other dogs, they all checked on him continuously, mouthing him, sniffing him, laying by him in his sleep, following him in the yard (he was deaf/blind), checking his crate in the mornings and at 3am when it was everyone else's time to go out they checked Teddy's snoring w/me. i didn't stop them or cut back their time. he was still a pack member, just the elder citizen. they knew it.
when i brought his body back from the vet wrapped in his crate blanket i didn't keep them away either. i took the whole pack out to the cemetery in the back yard where i have our other previous members buried. i had prepared his place before we left earlier. i laid his body beside his place. i finished deepening the bed, lined it with green leaves and flowers. while he laid there they all nuzzled the blanket, they opened it, they sniffed him, moved his body around, walked around him, tried to make him get up, after about half an hour they all just walked away. the only one that didn't was Grover, the little mixed breed i have that was very close to Teddy (the one that was put down). he lay down next to Teddy and rested his muzzle on his neck. i waited another 15mins or so but Grover didn't move. i had to take Teddy's body away from him and bury him. i put stones over Teddy's place and a cover over that. Grover lay down next to the cover. 20mins later i had to go out and carry Grover in the house.
it's now been just over 2wks and Grover is just beginning to recover. he's been grieving all this time w/depression and tearing up his skin and hair. yesterday he started eating normally again. everyone else recovered just fine w/in about 2days. but they did grieve, in a shorter time, and not as drastically as Grover.
i think you'll know, in your heart exactly the right day and time. you'll feel it. it'll just hit you at an odd moment and you'll suddenly feel at peace with the decision and the tears will feel relieving instead of painful. your pup may not grieve as heavily as my dogs did, mine are all adults and spent years and years w/Teddy. but he might miss him and wander around corners wondering where his buddy is, and look to you for guidance. be at peace about that. be at peace that there is the bridge and we once again will meet where there is no pain or tears.