I believe most sources use a 12 month time frame but that is no guarantee. You're right that the beta would be quite different depending on what time frame, what sampling rate (daily, weekly, monthly, etc.), and what update frequency you end up using. You'll want to check with whatever data source you end up using to be well versed in the inputs that your source is using. Different sources use slightly different inputs.

If you look up beta for a single stock across different data sources you'll likely get a different answer from each source. For example, Yahoo Finance lists MSFT's beta as 1.03, Google Finance lists MSFT's beta as 1.05, and MSN Finance lists MSFT's beta as 1.06.

I don't use beta for stock valuation myself so I'm not well-versed in how the various sites calculate their numbers.



Mike