Dog sneezed up a mess of blood. Was fine before and seems fine after. Eats and drinks as normal, no change in demeanor or daily habits.?

I rely on my Vet to call it done.

Right, why spend the time and money on an old dog?

He is not in pain. He is eating & drinking just fine. He eats his food. He is happy most of the time & just because he sneezes a little blood when he sneezes. He is not suffering. Personally, from what you have said, his time is not up yet.

Just going to say that when my 16 yr old dog did a bloody sneeze we took him in that day and didn't come home with him alive. Why? He had nasal cancer and because of his advanced age we decided it best to not consider treatments and not have him suffer. No other symptoms shown. From that experience I'd personally not ignore that and at least get it checked by your vet asap. Of course my experience isn't necessarily your experience with Your dog and don't throw away his life if it's not something terrible or easily treated.

Call the vet and ask advice. If the vet thinks he should be seen, then take him in. 

https://vcahospitals.com/know-your-pet/nose-bleeds...

This decision should be up to you in regards to whether you should take him in, but if he happened to eat any rodent poison, he would need to be treated immediately. A vet can likely advise in this manner. Rodent poison can kill a dog. 

It is time to pick up the phone and phone the vet and ask for advice.

The most common cause of an acute nose bleed in a dog will be either trauma or an infection in their upper respiratory tract...there are of course other reasons...however an URT infection can be treated very easily

No it's NOT time. There is no reason to put him down until he cannot walk and eat any longer. An exception would be if he starts bleeding again and heavily.

I don't own a dog but I still think you should take him to the doctor

A nose bleed in a dog is usually serious however if only a one off and he is showing no other signs of discomfort then I wouldn't be doing anything at this point.  If this happens again or becomes regular then you certainly need to look at the situation.  

Go by how he is in himself and if he's still happy and eating then it is not time to call it 'done' just yet.   You know the dog so be guided by how he is.

With mine, it's always been quality of life which dictates when it's basically 'time'.   And when they stop eating - and depending on whether my vets have anything more to offer (most relevant!).

None of mine has had a nosebleed - mouth yes, from oral cancer.    You could phone your vet for advice but if he's ok now, eating and happy, I'd just treat it as a once off?   Your call really.