Congestive heart failure is normally associated with the right side of the heart failing, and appears to be more common. You are right with that being associated with fluid retention. This isn't Mum's primary issue. She has been on Beta blockers for a very long time, and was only put on diuretics as a preventative measure against any fluid retention that may occur, should left sided failure, trigger right sided failure, and hence cause fluid build up.
You are correct about it potentially going on for years. However, given Mum's deterioration, and difficulty breathing, I don't believe it will be physically possible for her to continue for years. Sad, but true.

I've been told as much, but guess was just looking for a guage of time span should someone else have cared for a relative with the same issue.
Left-sided failureCommon respiratory signs are tachypnea (increased rate of breathing) and increased work of breathing (non-specific signs of respiratory distress). Rales or crackles, heard initially in the lung bases, and when severe, throughout the lung fields suggest the development of pulmonary edema (fluid in the alveoli). Cyanosis which suggests severe hypoxemia, is a late sign of extremely severe pulmonary edema.
Additional signs indicating left ventricular failure include a laterally displaced apex beat (which occurs if the heart is enlarged) and a gallop rhythm (additional heart sounds) may be heard as a marker of increased blood flow, or increased intra-cardiac pressure. Heart murmurs may indicate the presence of valvular heart disease, either as a cause (e.g. aortic stenosis) or as a result (e.g., mitral regurgitation) of the heart failure.