Ok, according to wikipedia:
- Regular Kalpa: 16,798,000
- Small Kalpa: its 1000 x a Regular Kalpa = 16,798,000,000
years [the age of the universe is relatively similar to that; about 13,000,000,000 years)
- Medium Kalpa: 20 x Small Kalpa = 335,960,000,000
- Great Kalpa: 4 x Medium Kalpa = 13,438,400,000,000
According to what Indrajala said, a Mahakalpa (the same as a Great Kalpa I suppose) is divided in 4 smaller kalpas: the formation kalpa, the existence kalpa, the destruction kalpa and the non-existence kalpa. And thats exactly the lenght of a Medium Kalpa to each one of this kalpas.
Since we are in the Medium Kalpa of Formation (about 335,960,000,000 years) and since there are 1000 Budhas on this Kalpa: Medium Kalpa divided by 1000 is 16,798,000,000 years the interval of time between each Buddha. Our universe is younger than the interval of a single Buddha, and yet we already had 4 Buddhas! Even so, the next Buddha (Maytreya) will come within millions of years.
By the way, here is the link to wikipedia:http://en.wikipedia.org/wiki/Kalpa_(aeon)