We hear this question all year! In a perfect world battery lights would stay on you know... forever. Unfortunately, the type of light bulbs, the size of the batteries and the quality of the batteries combine to make this question longer to answer.
As a rule of thumb, incandescent light strings with 10-15 lights should last about 3-6 hours on C-battery set. Microdrop LED lights constructed on thin uncoated wire should last over 100 hours with a fresh set of 3 AA batteries or 2 round C-style batteries and standard LED battery light sets with thicker insulation and LED lenses should last about 18-24 hours on a set of 3 AA batteries.
Keep in mind that battery lights will only perform as well as their batteries. Cheap, old or off brand may not provide the same results as a mainstream brand of batteries purchased at a high traffic store with lots of battery turnover.
Maybe that wasn't so short. Here's the ....
Pictured from left to right in the photo below: micro drop LED lights with 3 AA batteries, standard glass mini lights with C batteries, LED lights with 3 AA batteries and micro drop LED lights with C type batteries.
Micro Drop Battery Lights - About 100 hours
The set shown below is a set of micro drop LED battery lights manufactured with a battery box that works with 3 AA batteries. This set of lights lasts over 100 hours depending on the brand and age of the batteries.
These micro drop lights are manufactured with CR type batteries and last about 100 hours per battery set as well.
Incandescent Battery Lights - About 4-7 hours
This set of traditional glass battery lights has 15 bulbs and takes 2 C batteries. Since these incandescent bulbs use heating of a filament to produce light, they only last 4-7 hours per set of 2 batteries. They have the advantage of being economical and cast
LED Battery Lights
These LED battery lights with larger lenses and green or white insulated wiring aren't as energy efficient as the micro drop versions of LED battery lights but their larger lenses complement the lenses on LED light strings. So these battery sets work together with traditional LED string lights to create a cohesive look. These light sets last 18-24 hours per set of 3 AA batteries.