Of course, like you mentioned, there has to be a decline in value (preferably 80% or higher) - which signifies the ending of one price cycle and the beginning of another. But the thing is, with the ending of every price cycle - the next one becomes less and less likely to occur, or takes a little longer to occur... which is why accumulation cycles tend to become more extended over longer periods of time when you are dealing with older, more established coins.