iPad2 Battery Life Expectancy

My iPad2 will be 2 1/2 years old come the end of September of this year. We are leaving at that time on a combined six-week land/sea trip and I am concerned that the battery on my unit may exceed its life expectancy during this trip. If so, I will have no backup device along with me. I'm unable to find a good source of information as to what the life expectancy on the battery may be and I recognize that it can be highly user-dependent. Is there a source to provide me with some guidelines that would help me to determine if it would be prudent for me to replace the battery before we leave?