For ETFs, I’m a long-term buy-and-hold investor. I have an asset allocation plan with targets by asset class, region of the world, and sector, and I rebalance when they get too far away from my targets. But that’s quite infrequent.
For the part of my portfolio that is in individual stocks, I am currently doing the same as you: if a stock doubles, I have a strict rule to sell half to take profits, and then I reassess whether I think it still has growth potential, and either let it continue or sell it all and invest in a new prospect. I’ve had two stocks do that in the past few months. If a stock falls in price by more than 20%, I will reassess it and sell if I think it no longer has the potential that I once thought it did.
But stocks doubling is a rare thing! You wouldn’t think so from reading news about meme stonks going to the moon, but those are very much the rock-star minority. In the past 12 months of stock market recovery from the depths of despair we were in a year ago, everything seems to have been growing at double- or triple-digit rates, but this is the exception rather than the rule; the long-term average is more in the neighbourhood of 6% to 10% per year.
Slow and steady wins (or at least, finishes) the race!