Battery life of a certified refurbished laptop

Hi

Do you mean Hours per Day, or Years of Warranty?

Either way I would factor in the cost of a new one.

As %age of a battery in a new laptop. 

The average battery lifespan is 2-3 years so use the model you're considering as a guide.  If the model/system is a year old, it likely has about 2/3 of its life left.  That said, the system model comes into the picture as well -- business models stress the battery less than gaming models do.  And low-end consumer grade models generally have shorter-lived batteries.

As suggested by the the other reply, set aside funds for a battery replacement for anything over a year old when you purchase it.