"When do I test for my next belt?"
Sensei will test you when you are ready.  "Ready" means that you have met the requirements for 1) ablity to demonstrate
the required techniques proficiently, and 2) you have accumulated the requisite number of training days.  You should
take responsibility for yourself by seeking one on one instruction from sensei or a sempai to help prepare for testing.  
Usually you will experience one or two practice tests as sensei assesses your readiness.  
Example Requirements for kid ranks:

7th Kyu:  (Yellow belt)
20 days
Good knowledge of etiquette
Good behavior
Good attitude
Shows focus and discipline

6th Kyu:  (Orange Belt)
40 training Days

Tachi waza
tai no henko
tai no henko ki no nagare
morote dori kokyuho
tsuki kotegaeshi
shomenuchi ikkyo omote and ura waza
shomenuchi iriminage

5th Kyu:  (Purple Belt)
60 Training Days

Tachi waza
tenkan
tai no henko
tai no henko ki no nagare
morote dori kokyuho
tsuki kotegaeshi
shomenuchi ikkyo omote and ura waza
katate dori shihonage omote and ura waza
shomenuchi iriminage
Kid's Testing Information
Watch video of orange belt techniques at:
http://www.youtube.com/watch?v=kLwmkXcf0cs

Watch video of purple belt techniques at:
http://youtube.com/watch?v=fSYdvcLWwTY

Watch video of blue belt techniques at:
http://www.youtube.com/watch?v=p_YarRID2yg