The Avatar: The Game included significant detail about Pandora and the Alpha Centauri system that wasn't included in the film, notably that the 'day-night cycle' is highly variable and that nighttime on Pandora can be almost (10%) as bright as daytime depending on its orbital position.
The absence of discussion of Pandora's actual day/night length tends to suggest that it's about the same as Earth (i.e. approximately 12 hours of day and 12 hours of night, depending on the season).
Pandora receives significant light from Alpha Centauri B (ACB).
Because of this, for half the Polyphemian year its nights are never
dark, but instead are more like Earthly dusk. At the closest point in
its orbit, ACB is about 2,300 times as bright as Earth's full moon; at
its farthest, it is still one hundred and seventy times as bright.
During the other half of the year, when ACB is in the daytime sky,
many Pandoran nights are illuminated both by Polyphemus's huge disk
and reflected light from other nearby moons. Truly dark nights are
uncommon. Polyphemus occasionally eclipses ACB at night for about one
hundred minutes, but the light reflected by the planet still keeps the
night from being dark. When ACB shares the daytime sky with ACA, at
its closest it adds about half a percent to the total illumination.
When the 2 stars are close together in the sky, the effect of ACB's
more orange light is unnoticeable, but as they separate over the
years, an orange tint may be seen in areas shadowed from ACA's direct
illumination. At its most distant, ACB is about 2,700 times dimmer
than ACA and does not produce noticeable lighting effects. However, it
still appears as a blindingly-bright tiny orange disk in the sky.
Because of its high axial tilt (29 degrees), Pandora exhibits
considerable annual variation in the day-to-night ratio. In addition,
its elliptical orbit produces seasonal temperature variations and a
range in daytime illumination of about ten percent.