Basically how great can the effects of driftwood be? And how long does it typically last?
It is the tannins that help to lower pH; however, over time the tannins slow down and the pH begins to rise back to its normal level. I have something like 50 pounds of driftwood in my 125G tank. And the effect is negligible on pH. Honestly, I would not count on this for a long term solution. As for how long, that depends on how much of the tannins are soaked/boiled out of the wood before going into the tank.
I've had the DW for a couple months now and it doesn't tint the water anymore. I use turface which strips KH, so is this leaving the DW freedom to lower the pH as it pleases?
Without KH the pH becomes unstable. You need some hardness to avoid a pH crash. How much has the tank pH dropped? What is the pH of untreated water that has been aerated a couple of days? These reads with your GH and KH will answer your questions. Driftwood that is not leaching tannins will have no effect on water chemistry.
Also depends on your water change schedule. If you never change the water, then the effects (tannins/color) would be greater and would last longer than if you did weekly 90% changes Of course it's not advisable to never change your water.
My tap reads 7.6ph, 4 kh, 8-9 gh. In tank it's <6.0ph, 0-1 kh, 8-9 gh. This is with turface substrate.
Because the turface removes KH, as you've stated, it is my opinion this is why your pH has crashed. KH is an important factor in stable pH. Your driftwood is not the issue. You have a couple of choices as I see it: *Remove the turface and use a different substrate in your aquarium. *Begin dosing carbonate *Add crushed coral, seashells or eggshells *Do 50% daily water changes to see if you can maintain the KH level. Bottom line, find a way to maintain at least 3dKH. From what little I've read of your postings about turface and this product's website, I personally do not believe it is an appropriate substance to maintain a stable environment. I could be wrong, but there is nothing I have seen to indicate that it is inert or that it is appropriate to use in a water environment. Good luck finding a solution that allows you to maintain KH to stabilize your tank.