I would agree for the little wee ones that it is great just to have them going along and looking forward to class so if you can get them putting in another 15mins every other day or in the weekends that is great. I like the approach by them seeing what you (the parent) are doing (if the parent knows the style that is) and just getting involved and seeing it as fun, rather than pushing them at all.
Older than that, it depends in part what the focus of the club is.
For early teens onwards as long as it does not interfere with studies I would say an hour per night (or an hour in total per day) of extra/home training if they want to do well in kata and/or kumite tournaments. I never saw training as a chore and loved it from an early age and I would think for any dedicated young teen onwards that would be fine. Although they may have other sports they are also into and that require time – ah, you never realise just how much time you have on your hands when you’re a kid…
For mid- teens onwards into early twenties that are serious in doing well in tournament and want to get into nationals, an extra hour on skills per day would be the minimum and with base training (stamina/road work etc in morning before studies or at night) tagged on top of that as well.
That’s from my own experience in TKD and karate and from seeing the teens that take medals in my old club, they were pretty dedicated. When I competed while doing uni studies I would run in the morning, go to the club at least three times a week at night (1.5 – 2hr sessions) and then often a weekend morning session and do weights most days midday. If you love it, there is never too much time you can spend. You don’t want to over train but even when resting you can go through techniques, watch fights etc.
The comments that it depends on an individual are also correct to a large degree but if the focus is competition then no matter how inately good you are, you need to spend the extra time as that is what others are doing. Outside of competition, you also need to put the hard time in for getting good at techniques and understanding of the art.