While still working I did not spend much time with my investments. I did re-evaluate once a year or so. But then my performance was not very good. I did start long before the internet was available, today everything is more easy.
Now, you don’t need 100 holdings. For my mechanical dividend strategy 25 are enough. The important thing is sector diversification. I did write down all my rules for this strategy in the mechanical investment thread, feel free to pick any idea that you like there.
Now the momentum strategy is different. This did cost me more than 10 years of evaluating, trying, paper trading and so on. Countless hours invested, maybe over 1000, but it paid out. I did not start before I was sure to be able to survive with the dividend strategy and could lose all the money invested in the momentum strategy. That is probably the most important point. High risk, high reward.
I did not spend much time in selecting or researching stocks. I did spend much time in defining the rules for position and money management and stock picking. That time spent saves time (and money) on every single trade.