The length of a day varied between planets. The length of day for most terrestrial planets not subjected to tidal locking was in the range of 18 to 36 standard hours. Tide-locked planets did not have days. Satellite planets - moons orbiting a gas giant - were almost always tide-locked to the gas giant they orbited. Satellites may have days several dozen hours long (as long as it takes the satellite to orbit the gas giant).