What do you mean by "last?" Do you have wood eating plecos?
It doesn't degrade really, if that's what you mean by last, I know people that have had the same piece of wood for the better part of 15 years and its still going strong.
If you're wanting it to soften the water through tannin leaching that can last different amounts of time depending on your water. But I had one that I boiled the life out of and let soak for a while and it leached tannins for the whole year I had it before I got rid of the tank to move to Scotland.