History depth of Beta calculation

[m.runner]

Hi All,

What history period is used for beta calculation on yahoo and google?

For example,

here http://www.google.com/finance?q=ovrl
beta = 1.22

here http://finance.yahoo.com/q/ks?s=OVRL+Key+Statistics
beta = 1.38

Thank you in advance.

Regards, MRunner