Having been in this situation recently, as well as several times in the past, I know that there would be NO WAY for me to "wait until the last moment". Because, to me, the pet's emotional pain would be as real as any physical pain they might have.
A dog that can't walk............he doesn't understand why his legs will not do as he commands............emotional pain.
A cat who can no longer jump up on a favorite perch, no matter how low it is, will continue to try and fail..................emotional pain.
Any pet who begins to have constant accidents in the house.............they know it is wrong to be dirty.................they don't want you to be mad...................they know it puts a subtle strain on your relationship...........emotional pain.

Having dealt (in all instances) with cats, I know they hide their physical pain. The signs are subtle. They may eat less. They may stop doing a ritual thing that they have done their whole life. They may simply look "dull".

You will know in your heart and you will see it in their eyes when it is time.

{{{{{{{{{{HUGS}}}}}}}}}}}}