It is never an option to tell someone to buy bitcoin if they are a newbie and have no knowledge about what bitcoin is and what it does. If they know the fundamental of bitcoin and its usage, then they will obviously HODL till its value is high enough to give them the profit.
Don't ask them how long they can HODL. Instead teach them the basic of Bitcoin.
But in reality there are many professional traders who can't resist the urge to sell their bitcoins after few percent increase. Sometimes several things can disturb you psychologically about your belief in the potential future of bitcoin, including work, illness or so on. Holding forever also won't be very profitable for you because bitcoin prices tend to fluctuate once the cycle changes.
Beginners tend to rush when making decisions and that has made most of them fail to make profits or even have to suffer losses. The basics of bitcoin and trading must be learned as it is clear that it will form good psychology on the way. Obviously I don't think it's as easy as it sounds, but his experience and interest in continuing to grow strong will definitely make him more capable of making decisions.