It's 100% safe to carry a sleeping laptop around, for all practical purposes it's off.. it's just got a little power running through the RAM to keep it going.
A general rule of thumb is it'll sleep one hour for every % of battery life left. 100% battery charge, about 100 hours. If, however, the battery dies, the contents are also suspended to hard drive, so when you power it back up, it'll resume where you left it, in a few seconds instead of instantaneously.