Just like you said, a full circle which is 4 years can be slightly considered enough time for someone who is nearing retirement age to HODL depending on your financial strength and health status which will determine if they can go longer than a full cycle. Then for the vibrant youths who are full of life an energy, they can decide to HODL for more than a decade depending on when their target is met because as an investor, their is need to set a target for your investment so that when you reach your target, you can figure out whether to continue or not.
The dilemma is whenever Bitcoin price goes high people start saying it's too high to jump in now. Anyone who is new to Bitcoin must start from whatever price he see while investing and don't wait for the dip. Bitcoin has a cycle of 4 years and after every 4 years we see new ATH. To be on safe side invest for minimum of 5 years and that's how one can see full potential of Bitcoin.
Those who are young like in early 30s have golden opportunity in hands of investing in Bitcoin. They have time and can wait for more then 5 years, with time Bitcoin demand will go high while supply of Bitcoins will be decreasing with time.