How fatal is congestive heart failure?

Depends: It depends a great deal on how advanced is the condition. A patient who can walk a long distance with mild shortness of breath will do pretty well. A patient who has shortness of breath and leg swelling at rest have a 50% survival at 2 years.

Minimal to Rapidly:: Like most diseases, varies widely. CHF typically progresses slowly, years to decades; not recognized until symptoms=severe enough that people finally seek help (way down the road) due to shortness of breath (lung edema), leg edema, increasing fatigue, etc. Alternatively, if detected early (before symptomatic) & txed aggressively (NMR lipoproteins, HbA1c, BP, ACEI+Coreg) many significantly reverse.