Start with five minutes in the morning or in the evening. If that does not seem like enough time, then meditate for five minutes in the morning and again for five minutes in the evening. Later, when that does not seem like enough, increase your time little by little.
For the first month, the most important thing is to develop a sense of being at ease with yourself and having a good time. In the beginning, meditate less than you want to, so that you are always looking forward to the next session.
After a month, if ten minutes seems too short, then you can let yourself go a little longer. But do not meditate more than twenty minutes in the morning and in the evening until you have been at it for several years. It takes a long time to get used to being relaxed while in action, which is one of the main effects of meditation. There is a lot to learn about handling relaxation.
If you are really busy, even a few minutes of meditation is beneficial.
Excerpt from Meditation Made Easy book by Dr. Lorin Roche.