I was diagnosed with LUPUS in 1985...it is now 2004 and I am still kicking. In my case, I had no organ invovlement as I was diagnosed and started treatment quickly (by quickly, in the Lupus world, I mean within a year!).
The prognosis, as you said, depends heavily upon the symptoms and the type and extent of organ involvment.
There is no real "set-in-stone" time line for life span once a diagnosis has been made...there are far too many variables. You should also consider the fact that you may be one of the lucky ones who goes into remission!!
My advice, continue to take your medications, continue to make your health a priority, exercise, eat well, alleviate stress, and live a love-filled and loving existence!!
Peace and Blessings