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- How do you know an altcoin will be good for HODLing?
- How many years if surety do you have in HODLing altcoins and which altcoins are you currently HODLing?
1.
A bit of research, a bit of knowledge towards the project, a little bit of instinct, and a little bit of luck. 
Seriously, I've been investing into altcoins for a long time already, and excluding meme coins (because they are simply shitcoins), all of the altcoins that I invested with ends up giving me some small to huge profits. I mean ALL. My strategy? Don't sell at a loss.

If you need to hold it for years, and as long as you trust the project then do it.

. Kind of funny, but that worked pretty fine with me.
2. It depends. My strategy is that, I will always sell all of my altcoins during the bull run, so sometimes, I hold an altcoin for a year, sometimes in 2 or 3 years. Which coins am I holding? ATOM, TIA, and SUI composes a huge portion of my altcoin portfolio, and the others are just a small fraction (around 3-5%).
Overall, holding altcoins is very exciting because it's more volatile compare to Bitcoin. It will be more exciting if you will be holding altcoins that are at at least at the bottom 100. Of course always do your research before investing because anytime, these altcoins can just go down in price, and you might end up at a loss.