How long is a redstone night?
From the Minecraft Wiki:
Daytime
Daytime is the longest section of the cycle, lasting 10 minutes in PC and 6 minutes in Pocket Edition.
Sunset/dusk
Lasts 1 1⁄2 minutes in PC and 9⁄10 of a minute in Pocket Edition.
Nighttime
Lasts 7 minutes in PC and 4 1⁄5 minutes in Pocket Edition.
Sunrise/dawn
Lasts 1 1⁄2 minutes in PC and 9⁄10 of a minute in Pocket Edition.
A light sensor should output 15 for ten minutes, go down to zero over 1.5 minutes, stay at zero for seven minutes, and go back up to 15 over 1.5 minutes.