As the ones who posted above stated, meditation can be done at anytime and for any length of time. I personally practice meditation at least twice a day. As soon as I wake up in the morning for about 15-20 minutes I have my "wake up" meditation and before I go to sleep at night I have my "night-night meditation". These meditations are slightly different since the goal of each of the meditation is different.
Meditation is about being aware and thought flow. Many will say it is about thoughtlessness but I don't think that term describes meditation accurately enough. As you meditate you reach a state of awareness period. What you become aware of varies each time you meditate. During meditation you are not to hold onto thoughts, they are to flow. As a thought comes in you are to acknowledge said thought and then let it go. Many times a new thought will appear to take it's place and you repeat the process.
A comfortable sitting or resting position is the number one thing to help you to meditate. Music can help you to relax and keep you from dwelling on a thought for too long. A pleasant surrounding atmosphere is another big help. Many light candles and/or burn incense as they meditate. There are also many different types of guided meditations out there. Experiment with differnt things to see what works best for you.
Lastly don't be discouraged or become frustrated as you practice meditation. It takes time and practice to become skilled at it. Like others have said start with just a few minutes and work you way up to longer periods if you find yourself having trouble. Many Blessings.