Manufacturers will list their battery life on the product. Again how long you need the battery to last will depend on what you are doing. For head torches that have higher levels of brightness, the battery life will be shorter. What you really need to know is how long the battery will last at the advertised number of lumens. This information will usually be displayed on the packaging but be aware that battery life, unless stipulated, refers to how long the light will last until it finally dies and won’t necessarily be at the advertised level of luminosity.
In addition to battery life you’ll want to decide whether to choose a head torch with rechargeable batteries. Where possible this is definitely a good choice. A rechargeable lithium-ion battery, if well made, will always be much more efficient at pumping out power than standard alkaline batteries, especially at low temperatures. Also if you are a regular user of your head torch, then you will quickly go through batteries, it’s often cheaper to buy a spare rechargeable battery than to keep buying standard batteries. The only time I might not use a rechargeable torch is if I’m on a remote expedition and have very little access to solar power to recharge.
There is one last additional feature that is new to headtorches and which can help to improve battery life and that is reactive lighting. This means that the headtorch detects incoming light and adjusts the brightness accordingly, thereby saving battery power. Neat.