You'll know it's done when it's not as resistant to stretching. If you have more than one balloon, you can use them to compare.
This is when the stretching comes in handy. Hold the bottom of the balloon in one hand, and pull the neck of the balloon up with the other.
Keep pulling the neck and pushing the rice down. Add rice to the neck every time you push the rice back down. Make sure to fill the tip of the balloon!
Do this until you get it to the size you want. This rearranges the rice so you can fit more rice in, and stretches the balloon to give you more space.