Battery life... you mean as reallly battery "life" or as in how long does the computer run from the battery while using it?
I bought my MB on May and iStat Pro says my battery health is 91%, my MB runs from the battery everyday and i charge it everyday. So i guess it's really good.
Now... if you want to know how long does it run from the battery... well, depends on what you're using, i.e. if you're watching a movie the battery won't last long and you'll have to plug your MB, but for documents, web surfing and chatting you can use it around 2 and a half hours