The disease Lupus can be serious if you leave it untreated but if you get treated for Lupus you don't always die soon.
Having Lupus does not always mean that your life will be shorter although it can be if you don't get treated for the lupus and let the lupus disease progress.
There are many people who have been diagnosed with Lupus and are living a normal lifespan and some people have been living with the Lupus disease or more.
My Grandma lived with Lupus for 50 years even though her doctor at the time said she would probably only live another 20 years after the Lupus.
But she was diagnosed with Lupus before I was born and the advances in medicine and doctor knowledge wasn't as great as it is today.
So if you get treated for Lupus it doesn't mean that you'll have a shorter lifespan but it's still possible and there's really no way to say or know 100% for sure how long you'll live after being diagnosed with Lupus.
However you should live 20 years or more after the Lupus diagnoses.