If I have invested money in altcoins then I never hold them too long and sell whenever its giving me at least some profits as it can be bought again when price dumps and bitcoin is an long term investment for me so I will not sell till it achieves my desired price and till that time I will keep them safe in my wallet.
yes i will also have the same strategy, and i will also never like to hold altcoin for a long time, but will surely sell it when its price start increase a little, but if i am trading altcoin then i will surely hold my bitcoin for a long time because i am sure that bitcoin investment is the best investment.
Sometimes i do hold altcoin for the long term ,
For example last year when i bought dogecoin for 20 satoshi only and have sold it all at 130 satoshi ,
Guess how much i have made money from it? Quite a lot right?
You have to be patient , and then you could sell anytime!
there is no other better way to decide the time to sell your coins but if you have made up some margin of profit in your mind. I think if you have already decided that you want to make out this much amount of profit at the time of buying the Bitcoin, then you will have no worries at the time of selling.