So I keep hearing HODL, HODL, keep holding and all that. My simple question to everyone is "when is actually the right time to sell Bitcoin or we don't ever sell it"?
You don't have to sell your bitcoins as they're money. You obviously have to spend them sometime, but not necessarily to exchange it for fiat. Most of the hodlers believe that it'll someday be the global reserve currency in which everyone will use for their daily transactions. They'll have a head start based on the assumption it will indeed achieve this level of adoption, because they'll have increased their purchasing power by a lot.
Also, bitcoin has managed to always get over its ATHs, so it'd be wise to keep them instead of spending them as they seem to worth more in the future. It is uncertain to know exactly when it's ideal to spend, but if your purchasing power was a function, you'd have to let your
time parameter be as great as possible to reduce the risk of spending them in an undervalued price.