Depends on what altcoins you are holding. If you are holding shitcoins which is for pump and dump then you should sell when you think that it has reach the top. But if you have holding a good altcoin specially in the top 20, then there is still a chance that it can go even higher as we have just started the altcoin market rally. And as long as bitcoin is on a bull run, these altcoins are going to enjoy their all time high and surpassing it this year with new one.