On hardware for which high-quality drivers exist, GNU/Linux has the potential to outpace Windows and OS X[0] by a decent margin. The OS and kernel themselves are very power-efficient. You're not going to double your battery life or anything, but you might get a noticeable amount more. It depends a lot on what software you're running and whether you're taking advantage of any power-saving features that GNU and Linux provide.
However, unless you've chosen your hardware carefully, GNU/Linux will give you slightly worse battery life, because the drivers may be less efficient than whatever the manufacturer's native (Windows) drivers are. Again, the difference is significant statistically but not practically.
[0] The latter is a bit trickier to compare precisely, because OS X is designed specifically for very closed-source hardware for which no decent Linux drivers exist.
To put that more bluntly, Linux battery life entirely depends on the quality(1) of the Linux kernel modules for your hardware, and those vary wildly.
Yes, what Linux does in software is a lot better tuned in many places than the equivalent parts of OS X or Windows (in case of filesystem performance, sometimes frighteningly so). None of that matters when some component in your system doesn't properly enter its sleep states, because that component will suck your battery empty.
On the flip side, if you research Linux hardware support before buying your laptop, you should get very good results.
(1) Edit: I don't mean poor quality of implementation, just incomplete functionality. There's little the module maintainers can do about poorly documented hardware.
However, unless you've chosen your hardware carefully, GNU/Linux will give you slightly worse battery life, because the drivers may be less efficient than whatever the manufacturer's native (Windows) drivers are. Again, the difference is significant statistically but not practically.
[0] The latter is a bit trickier to compare precisely, because OS X is designed specifically for very closed-source hardware for which no decent Linux drivers exist.