I think the best is to have a plan and stick to it. For example, you can pick 2 - 3 coins or more and invest some money in them.
If you want to invest long term, it is easy, make some offline addresses (ex. paper wallets) send the money from the exchange, and forget them (except you private nr).
Then you can decide to invest (let say $100 each month) if later you find a new coin interesting, and read a lot for it, just buy it and you'll have one more in your portfolio.
The question, how long to keep the investment, depends on your needs. The majority says 5 to 10 years.
I say if your investment is enough to buy a house and you live on rent, then buy the house, it is a better investment.
I also believe that if you can invest in real estate that is safe option then crypto.
But the problem is of course is to have that amount of money to invest in it.