You should be mining new altcoins only if you believe that they will have a great future ahead of them. I personally do not mine these coins but from what I have heard here, people that do mine these kind of coins do so for a minimum time of 24 hours to 7 days, depending on the rewards you make daily of these coins.

If a coin is so new that it gives you like 50000 coins daily , mine it for only 24 hours, if it does give you like 200-300 daily, mine it for a week.