As a recommendation it usually takes 20-30 minutes to meditate. It can take up to 40 minutes or even an hour as you develop yourself. There are monks and spiritual people who can meditate for days. Or we know the people who lived in deep meditation for days. We can not talk about the time limit, but the ideal meditation is between 20 and 30 minutes.
It is useful to show that it is quite difficult to make 20 minutes in the beginning. Even if you have no time, you may not be able to keep up. Do not force yourself. As you do regular meditation, you will reach 5 minutes first, then 10 minutes, then 15, and finally 20 minutes. Meditation is not done at once, but it develops gradually with time, so do not forget it. So let's get out of your mind for a short while.
When I sat in my first meditation with great enthusiasm, the hours seemed to have passed, and I realized that it was only 5 minutes when I opened my eyes. Not only that, but my feet were numb and I could not get up from the earth for five minutes. I mean, I could not catch the first time around, it was not efficient. As long as you do it later, all these problems are behind you. Even in your meditations, which sometimes take an hour or so, you may even feel that you have been feeling as short as a second.
So, please do not break your heart if you can not do it for a long time in the first times, not to say you do not succeed, a big 5 minutes but you can barely stay in meditative position at first. Over time, as I said, it is going to higher time limits.
One and the meditation does not take place, you learn to meditate constantly and this is a learning please. Continuity is very, very important. It is important to meditate regularly every day.