Yeah every Ultraman has a color timer so they can know how long they will stay on a specific planet, so it's like this. Due to Earth being polluted and the Sun (Stars like the Sun are the main source of power of Ultramen) being too far away from earth, Ultraman cannot absorb enough energy to withstand the harsh conditions so that's why they only have 3 minutes but if their in space they have much more time.



Fact: Ultraseven on the other hand is much different, you see Ultraseven has a Beam Lamp instead of a Color Timer and it's the thing that tells him how much time he has but the difference is Ultraseven has that armor thing around his chest which absorbs Sunlight like a Solar Panel that stores energy. So Ultraseven can stay longer than any other Ultraman and the Beam Lamp will only flash if he takes too much damage or he's really running out of energy.