About a replacement battery

I am thinking to buy a new battery for my Unibody Aluminum Macbook late 2008, and i have one question: this battery would give me 3 hours (as the original one) or 7 hours (as the new models batteries) of working time?
I suppose that it will be only 3, but this is something not totally sure for me.
Any help, please?

flatfoot said:

Assuming the quality of the replacement battery is OK, battery life will be the same as with the original one.

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That's what i supposed. Thanks.