I always think of purchasing bitcoin as buying a lottery ticket, or gambling. One should be prepared to lose every cent, so only invest what you can afford to lose.

I think it makes sense to sell just the amount of your initial fiat investment, once the value has grown significantly. That way, you didn't lose any money on your bet.

Then, if some other technology comes over the horizon that looks like it's going to displace bitcoin, trade the rest before too much of the value is lost.