This longest division of geologic time doesn't have a fixed period; in astronomy, it's one billion years
The 
major periods in the geologic history of the Earth are (mya=million years ago): ... 
2.5 billion years ago to 540 mya, -, Vendian 600 to 540 mya ... Astronomy [DOC] [TLE] Radiometric Dating - American Scientific Affiliation [PAR] However, some Christians suggest that the geologic dating techniques are .... 
Radiometric dating of rocks also tells how much time has passed since some 
event ...