Time period between the use of dragon balls during Namek Saga
According to the Dragon Ball's timeline's wikia page: 


November 2nd, Age 762: Goku is revived on November 2nd, Age 762 by the Earth Dragon Balls
Age 762, December 24: Every one killed by Frieza on planet Namek is revived by the Earth Dragon Balls


So (much) less than one year has passed between these two uses of Dragon Balls. I don't know if there is a canon explanation for this, but maybe the death and resurrection of Piccolo and Kami reset the Dragon Balls. This is the only explanation I could think off.