Shortly before the end of his earthly life, the Buddha said to Ananda (his disciple):
"I am not the first Buddha to come upon earth; nor shall I be the last. In due time, another Buddha will arise in this world, a Holy One, a Supremely Enlightened One, endowed with wisdom, in conduct auspicious, knowing the universe, an incomparable leader of men, a master of devas and men. He will reveal to you the same Eternal Truths which I have taught you. He will proclaim a religious life, wholly perfect and pure; such as I now proclaim."
Ananda then asked how we will recognize this future Buddha, to which the Buddha replied:
"He will be known as Maitreya which means kindness or friendliness."
Buddhists sometimes wonder when will Maitreya finally arrive. After all, the time is probably quite ripe. When Gautama Buddha was around, the world was in Bronze Age, and he taught people according to their level of understanding. Nowadays, we are in the Information Age. Many of the things the Buddha taught 2,500 years ago make more sense to us with our knowledge of modern physics as well as psychology. So how much more could we learn from a Buddha who would restate the dharma in a modern way.
To answer the question of when, we must be reminded that a Buddha is not some Avatar sent by some deity when the time is ripe. A Buddha is a person who has decided to come to a full understanding of the nature of reality, and then spent many lifetimes working on that understanding (see the Bodhisatta node for more on that).
Hence, Maitreya will come when Maitreya is ready to come, not necessarily when we decide it was about time for Maitreya to come. Indeed, he (or she - there is no reason why the next Buddha could not come as a woman) can be any one of us. Since the Buddha is not sent "from above," Maitreya will come when someone seriously decides to become a Buddha and starts working on it. Let's just hope that someone already has decided and is working on it, though there certainly is no reason why you or me could not make that decision.