There's 2 types of survey sensors, gravity and geological. Grav surveys are for the jump point survey locations scattered around the system. Geologic surveys are for planets, moons, asteroids and comets. I think when you send a ship with geologic survey sensors it will display that message "survey points required" and a number, that number is larger depending on the size of the body. Mars for instance requires 340 survey points. A standard survey sensor provides 1 point per hour, so just advance time and you'll see the required survey points go down.