Change it after every bowl, and DO NOT be lazy and leave the water in there. If you do your bub is going to smell/taste horrible very quickly. Also, just poor the water in through the bowl and wait 10 seconds for it to try.
Fresh water every time. If I'm feeling extra lazy, I might let it go for two sessions, but that's rare. When I weigh my laziness versus the ungodly smell of the water, it's just a no brainer. For the sake of personal hygiene, change the water every time.
good thing i have a pull bowl bubbler, so it is easy to change the water. I usually change mine pretty often, but i don't use it that much, only when i am at home and i am mostly at my friends smoking. Once my incredibowl mini gets here my bubbler will be obsolete.
Dude, it takes, what, 10 seconds at most to empty the water out of a bubbler. It's not exactly difficult. Obviously if you're smoking it all day you're probably better off changing it at the end of the day, but it's not exactly a chore.