Another cracking quote from 'Unstoppable' - http://www.unstoppable.net/
"After you've done a thing for two years, you should look at it carefully. After five years, look at it with suspicion. After ten years, throw it away and start all over." - Alfred E. Perlman
In management and leadership settings in business I would endorse this statement fully - and I would say make it far less than 10 years before throwing it away and starting all over - more like 5 years maximum in my view.
In love and relationships I don't agree with the statement at all. Finding someone special that you want spend the rest of our life with means for ever.