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.
Yes, that's correct, you should only mine coins that have a "future".
It also depends on the strategy you want to use. Some people prefer to mine & sell right after on the other had some mine & hold for when prices increase.
That is the hard part the "future". I've heard a certain amount of time too, like you said 24hrs. What strategy are you using and is it working for you?