There are so many variables there that I'd wager that it's impossible to say with any accuracy. How much of it is removed by vacuuming, for example can have a huge impact. All potting soil is not identical by any means too. I have been breaking down and re-scaping a couple of tanks that I set up around 2008 with dirt, and still found some there. The plants were thriving but was the old soil doing anything at this point? And at what point does the nutrient load in the soil start to decline and how sharp is this decline? I'd say it's different in every tank, and the soil used (and what amount) would be a huge variable. A jungle tank with tons of plants would surely expend it faster than a light planting of undemanding plants would. And one would think that how much fertilizer you add to the water would have some effect on the rate of depletion also.
Nothing good happens fast in an ecosystem.