If we continually invest in bitcoin for, hypothetically speaking, probably about 10 or 20 years. Is that timeframe sufficient enough to sell bitcoin? Its different from stocks as it does not give dividends and is not like real-estate that helps you preserve wealth as the movement of it fluctuates and is very erratic. Ive also seen some posts on other forums talking that in the far or near future, bitcoin would become a common medium of exchange and would further increase its purchasing power? I find it speculative but its also plausible.
The right time to sell can be subjective. In my personal opinion, perhaps I'd buy a luxury item like the latest Lamborghini or Ferrari (lol), or perhaps I'd do it if I wanted to donate to an orphanage or disability foundation in the future, which wouldn't be a bad idea. Everyone has their own goals when selling their Bitcoin in the future. There's no set timeframe; it depends on personal preference.
Whether or not Bitcoin will be used as a means of transaction like FIAT is now, I believe it will become a superior store of value.