Does anyone know in which era each of the first 27 Buddhas appeared? I know the 28th Buddha's lifetime is generally accepted to be circa 563 BC to 483 BC.
Gautama Buddha, the traditional Buddha is thought to have lived towards the end of the 5th century BC, about 480-400 BC. This date is the scholarly consensus. Traditional groups accept other dates, and there are a number of these.
The previous Buddhas are a myth. By which I mean that no one thinks that they actually lived. The previous Buddhas were probably an invention along the lines of the Jain myth of the Jinas. The Buddha's mentioned in the Pāḷi Canon lived in the universes that preceded this one - an incalculably long time ago. This is based on the idea that the universe goes through cycles of creation, evolution, devolution and destruction over incredibly long periods of time (much longer than scientists give for the present age of our universe).