When it was new, my iBook 12" burned about 3%/10 hours sleeping, so that it would last a week or more, in principle, from a full charge. So a long time.
If you want a more exact number for your laptop, you can download something like
XCharge, let your laptop sleep for a while with it running, and then use a calculator to extrapolate a very accurate answer quite easily, right?
But if you are saying that your notebook, with a full charge, fully asleep for one day continuously, is at or near empty, that isn't right at all. Either it is not going to sleep correctly, or you have something wrong with your battery.
Are you sure it's asleep?