It's nice to be prepared but I would say, don't get ahead of yourself.  Like Jim said, enjoy each day as it comes.  When things change (and they will), you'll deal with it.

I know your fears because I have shared them — but the predominant advice is good: enjoy each day. Sammy, who also had osteosarcoma, recovered beautifully from the amputation of his right front leg last November. Within weeks he was running, jumping and swimming. We opted for chemo; in fact, we did a sixth round after we heard that some university sites were reporting good results. We tried artemisinin , a herb supported by great anecdotal evidence and the subject of a clinical trial at Ohio State university, but it aggravated Sammy's epilepsy so we had to stop. After our intial worries, we settled into a state of gratitude and the feeling the Sammy would beat the odds. Every day was special and we made sure we enjoyed Sammy and Sammy enjoyed life. Every checkup was a victory when his lung x-rays came back clear. Then, this September he collapsed while swimming and x-rays showed cancer ravaging his left front leg. Ironcially, his lungs were still clear. We decided to do nothing more than monitor the damage and make Sammy happy. We, too, worry about knowing when it is time. These past two weeks have been difficult. He no longer walks outside by himself, although a Thanksgiving houseful motivated him to walk up and down the stairs of the deck by himself. Today he was exhausted, and my husband had to carry him outside again. Despite it all, his personality is intact and his tail thumps joyfully. He tosses his head and moans for attention, then grins when he gets it. He still makes us laugh every day and he doesn't seem to be in pain. I don't know when he will no longer enjoy life, but our vet gave us some good advice: she said to not panic at the first bad day, or even the second. Three bad days in a row probably means it is time to let them go. So, for now we are in a holding pattern. Sammy's life is not perfect, but I thnk it is still good.  It would be so much easier if they could talk, wouldn't it?

To answer some of your questions - just because it is in the lungs doesn't mean it's "time." Abby's first very tiny met was spotted last December. She is now up to 3 mets, so hers have been slow to develop. (We are worried now the cancer may have spread to her remaining leg, but that's a whole other recent story.) But... in the meantime, she's had many many months living with her lung mets - and being VERY active with them. They don't cause her any trouble at all - at least not at the moment.

Try to not worry about that they will necessarily develop - maybe you will be lucky and you'll have worried for nothing. (I know that's easier said than done - esp'y from me the queen of worrying.) If they do develop - you could still have a lot of good time left with Levi.

Hang in there. Cancer's a rough ride.
Jackie, Abby's mom