Hi! Based on expert reviews as well as Sony's own battery life rating, we have the following information on the Sony Xperia M's battery: "The removable battery is rated at around 9-10 hours of talk time."
And as for the warranty, it should be covered for the same amount of time that the phone is. If you purchase a new battery unit separately, it is covered by a separate warranty period.
We hope that answers your question!