I got a small (prolly holds 1/3 cup of water) glass-on-glass and I change the water about 1/week. I use new water about two to three times a week. It's a clear bong and bowl, so when the resin builds up, it looks so nasty. It just looks soooooo nice when it's fully clear, but the bowl is hard to clean, even with q-tips.
Instead of 420, I've used "Bling" I bought from a flee market. It smells sweeter - like sugar instead of salt. I think it cleans a lot better than 420. The Bling works great for small pipes too. You just put some in and plug the carb., mouth, and bowl, shake for like 10 seconds, then repeat if necessary. It's an arduous process, but rewarding if you own a cool looking piece.
clean pieces > nasty gunked up pieces (although, I hate sucking green bud through when there's no resin to clog the hole a bit.)