It's personality-based, occupation-based, people you live with based, all sorts of things like that. You've got to make a decision, morning or evening. For a lot of people the morning is a time they can fit in easier than the evening if they live with other people, because the evenings... you know about that, I don't have to explain it.
"For how long?" Let's get into the words, "how long". A regular practice is better than a "how long" practice. This basically means that if you do five minutes every day, every day, every day, every day, it's better than trying to fit in an hour whenever you can, because an hour often does not fit in at all, maybe once a week or so. So be realistic with the time that you have, and try to set something that you can definitely do every day, whether it's five minutes or fifteen minutes, something you can always fit in. And be very realistic and objective with it. If you solidify, if you can make it a practice over and over, whatever amount of time, then at some stage you can say, "Oh, I can add another five minutes, I can add another ten minutes to that. I can fit in more." But if you've never ever made it solid and you're just sporadically doing an hour here, and hour there or whatever, then it's not going to happen that you ever want to increase it on the days when you're not doing anything anyhow. So, if you're doing a little bit every day, every day, every day, that's guaranteed, and you promise yourself you'll always do that, it doesn't stop you from expanding that on one particular day or two particular days. Maybe you say, "I'm going to do 20 minutes every day," fine. Some mornings you may wake up and you know that you've got nothing to do, you're totally free. Well why not do an hour if you've got that much time, why not? But if you do a minimum 20 minutes (or whatever your set period of time is), then you may discover that you're going to expand it as the months or the years go by. So a regular practice is more important than a "how long" practice.