How often do i need to change the water

How often do I need to change the water, if at all in a wc setup? How will I know when to change the water?

i'm going on 18 months without changing mine. works just as good as the day i put it together. if you see little creatures swimming in the tubes you'll know it's time for a change

but i think i'll change mine in a day or two since i have a week off of school

: ) :

Put something poisonous in the water like antifreeze and you should never have to change it.