The doctors will tell you it is not life threatening.  I'd say it depends on the definition.  I have had FM (fibromyalgia) for almost 20 years now and have tried every treatment and finally found a good pain doctor and a way to live a full life with what pain me and my pain doc can't lick.  My mom started having FM symptoms 5 years ago, and was diagnosed about a year after that.  The unrelenting pain surprised her, she'd lived a blessed life and had nothing really painful before that.  As a result, severe pain was something she just couldn't imagine.  For years she had been really nasty to me if I said I needed to do something (see a Dr, take a pill, rest a bit) because of my FM pain.  Once she had it she talked of nothing else, laid in her bed 24 hours a day refusing to do anything because it might increase the pain, and just slept, ate a little and took the constantly changing meds her well-meaning but unfamiliar with FM doctor gave her.  She died 2 years ago this month at the age of 70, had been in perfect health 3 years before.  Her parents and grandparents had lived to be close to 100, so she didn't have a family history of early death.  She died because she quit living because of the pain of fibromyalgia.  I'd call that dying of fibromyalgia. 

As I tell my doctor at every visit, I'm not my mom, but I understand what happened to her and why, and why she chose not to go on with the pain.  It's why I insist my regular pain med keep me between 2-3, rather than the usual 3-4, and why I take my breakthrough med the second my pain climbs above 4, or even stays at 4 for more than a few hours.

October is hard for me because she died in October and I have such mixed feelings.  I was so angry because she dismissed my pain for so many years and so many nights I wished she could understand my pain.  I know I didn't cause her to get FM by wishing she understood, but I sometimes wish I hadn't wished it.  But this is off-topic and doesn't answer your question.

FM doesn't have stages per se.  It sometimes waxes and wanes, when you have nearly symptom-free periods punctuated by severe flare up of symptoms.  Other people (like me and my mom) have symptoms pretty much always.  Just as there are two main schools of thought about the cause of FM, I'm betting there's two seperate diseases, and someday we'll be able to distinguish between them.