Recently I noticed, my battery does not last more then 2-3 hours when not plugged in. When I got the Mac it was around 5 hours. Of course this is development usage, typically with Eclipse+Jetty+Browser+Airport+Adium+iTunes running.
I usually never shut down my Mac. Just close the lid. Also, it is plugged in round the clock except for maybe 5 hours daily. I googled around and got conflicting opinions on exactly how the charging patterns should be.
Also, it is Summer in India right now with temps going up to almost 35-38 C. Would it affect the charge?
I've found that the only way to keep my apple products from gaining a lousy battery that can die in only a few hours is to shut it down every few nights and let it restart and not plug it in when the battery is rediculously high (60% or more). My computer usually never gets that low due to frequent/constant usage during the day but it usually takes me 7-8 hours of usage for it to die. I would recommend bringing the brightness of the device all the way up or playing music off it or doing any other battery draining activity for the 30-60 minutes before you know it's going to charge just so that it starts it's charge as low as possible.