Many beginners to meditation ask what is a good length of time to meditate for. The answer depends on the seeker and the inspiration they feel. Generally, it is not easy to give a particular time period; but, in the beginning, it is good to aim for at least 15 minutes.
If you meditate for too long, the mind will start to rebel; it will become very difficult to control your thoughts and you may feel a pressure in the third eye, a little bit like a headache. For beginners it is important not to pull beyond their capacity. When we try to pull in too much spiritual light, we cannot hold it. Similarly, if we meditate for too long, we will just feel a barren desert and derive no benefit at all. At this time, we would be better served by reading spiritual books or chanting mantras.
When we have been meditating for a few times, we will soon be able to tell when we are really meditating, and when we are just sitting there for the sake of it. If we just sit for a long time we may go through different cycles of thoughts and not get anywhere. When this occurs, it is a good time to stop.
We should always remember that we meditate for our own benefit. What matters is our sincerity. Nobody is watching over us giving us a reward if we meditate for a long time. We should always feel we are meditating to feed the inner pilot and strengthen our inner capacities. If we start to feel proud and pleased with ourselves for meditating a long time, we should no this is not a good sign. Real meditation will never give us a feeling of superiority; instead we will gain a feeling of oneness. If we sit in meditation for a long time and feel bloated with spiritual pride, this is a sign that we are meditating too long. As Sri Chinmoy suggests
“It is not how many hours you meditate but how you meditate. If you can meditate soulfully and devotedly for fifteen minutes and sincerely cry for Peace, Light and Bliss, that is better than three hours of meditation without any life in it. When you meditate, if there is a living presence in it, then that is meditation. Otherwise it is no better than sleep or death.” 
Another factor to bear in mind, is that when we sit down to meditate, in the beginning our mind will be all over the place. This is because it takes time for the mind to calm down. Therefore, if we meditate after coming home from work, the first 10 minutes may be very unproductive. However, after the initial period of 10 minutes, we may find that we start to really meditate. Thus, we may need to set aside 20-30 minutes, in order to meditate well for 10 minutes.
When we become more advanced in meditation, we should try to advance the time spent in meditation. If we feel an inner urge to deepen our meditation and find the real silence within, then we should definitely follow this. To attain higher states of meditation it is necessary to develop the capacity to meditate for a long time of 1-2 hours. Adepts in meditation can meditate for several hours at a time. However, in the highest state of meditation, we lose track of time; this is because the highest meditation transcends concepts of time. If we are able to enter into the highest state of meditation, we will become completely unaware of time, it will no longer be an issue.
However, we have to start from where we are. To meditate soulfully for an hour maybe our goal; but, in the beginning we need to focus on the quality of our meditation, rather than the length of time.
 Excerpt from Meditation: Humanity’s Race And Divinity’s Grace, Part 1 by Sri Chinmoy.
By: Tejvan Pettinger. Tejvan is a member of the Oxford Sri Chinmoy Centre and gives free meditation classes in his home town. Tejvan has studied meditation under the guidance of Sri Chinmoy for 8 years.
Photo from: World Harmony Run site