Buddha's appear only during a time of decrease within a Kalpa of duration, when the human life span is between 80 000 and 100 years.
There are seven Buddhas of the Past which are noted within the Pali cannon.
Names and Period of Existence
Vipasyin 91 Kalpas ago
Sikhin 31 Kalpas ago
Visvabhu 31 Kalpas ago
Krakucchanda In the present Kalpa
Kanakamuni In the present Kalpa
Kasyapa In the present Kalpa
Sakyamuni In the present Kalpa
According to research done in Japan based on a Great Kalpa (mahakalpa), one of these great kalpas consist of a changing cycle of four kalpas, they are namely: (1) Kalpa of dissolution (samvartakalpa), (2) Kalpa of nothingness (samvartasthayikalpa), (3) creation kalpa (vivartakalpa), (4) duration of a created world kalpa (vivartasthayikalpa). Each of the four kalpas last twenty intermediate or anatarakalpas. One great kalpa consist of eighty kalpas. A kalpa is sometimes said to be 4.320 million human years.
Based on this research, they concluded that Maitreya Buddha, the Buddha of the future is due to appear in 5,670,000,000 years after the Nirvana of Shakyamuni Buddha.
They also stated that for one to perfect the paramitas and rid of every dust of klesa would take three incalculable (asamkhya) kalpas or if thought of mathematically "3x10^59".
This is based on a class I took at my University, it is quite academic, but that's what we were taught.
To be honest though, I like this quote a lot better than what I just stated:
Shakyamuni is quoted as saying that wherever sentient beings suffer the Tathagata arises