I'm not a car expect, but in my opinion that depends on your car. How much you drove, and what speeds you drove in.

For example, if you're driving a Hyundai Getz (VERY COSTLY IN GAS), and you drove 60 kms in the city, you need to refill the tank because its half empty already.

If you drove a Nissan (More economic in gas)  in the city, for the same length, the tank of gas will show full, because you didnt spend much.

If you drove the same Nissan out of the city, meaning in higher speeds, the tank of gas may still show full after you drove 100 kms (i dont know if you calculate in miles, multiply kms by 1.5) 

So your question has a lot of variables, it may not be the problem of your gas guage

If you drove enough to know for sure that the indicator should go down, i suggest asking a mechanic ;)