The perfect time to sell is only when you absolutely must.
Wouldn't you rather be holding a nest egg of Bitcoin rather than a depreciating asset like fiat currencies?
We could argue about that.
Here's some possible answers.
1. The best time to sell is never. Many of us are not traders and we do not buy BTC, but earn it. Some of us would prefer to spend it then sell it.
2. Sell it when you are at a life-changing profit. For one of you it will be a profit allowing you to buy a dream car, for another to start a company, for someone else to start a business.
3. Sell it when you are certain there will be no regrets. If you feel like another 10% might be important for your financial future, don't sell it.
4. As long as you have fiat money and nothing to spend it on don't sell. Don't go all fiat just because you're scared, or because you feel like fiat is better, that's dumb. Go fiat when you have something to spend it on.