Earlier versions of Fiberflies use AAA batteries. Since 2016, Fiberflies use Li-ion 18650 batteries.
If your FiberFlies uses AAA batteries, 1.5V alkalines should run at full brightness for about 4-6 hours, after which they are still pretty well lit, just not as noticeably vibrant. Typically you would need to replace the batteries every two nights of active usage. With extended use, you might need to change batteries every night. The white mode uses the most power; all other modes draw a bit less.
If your FiberFlies uses a li-ion rechargeable battery, it should last between 6-8 hours depending on the mode.
Here's a handy link to FiberFlies PixelWhips: http://www.flowtoys.com/pixelwhip/
If you have any questions, please feel free to contact us!