LED needs a minimum current or minimum voltage to operate in forward bias
Taking a look at a datasheet for a popular IR LED, the Lumex OED-EL-1L2, we can see the IR light output vs. current curve: 



So if you only give it 1mA, it gives you 0.2mW/sr radiant intensity. At 20mA, you get 10mW/sr radiant intensity. 

So, to maximize battery life, you should use the minimum current that will produce a light intensity that your circuit can (with an appropriate safety margin accounting for all the things that can change, including LED aging and temperature) work with. 

If the sr unit used for the radiant intensity is unfamiliar- that's the SI unit of solid angle, so it's power per unit of solid angle.