Late to this, but good to know as I've been wondering the same thing.
Sites that I find helpful (like "The 24 hr Aquarist") say that after 18 months a tank with Aquasoil needs a "reset". I don't know why the option of adding root tabs isn't mentioned. Is that what you do with your long lived tanks?
Truly depends upon your setup. There's no set lifespan or period of usefulness. If you have a heavily planted tank, you'll probably use up nutrients faster than someone who just has a few Cryptocoryne and and some epiphytes. The heavily planted tank may need supplements after a couple years and the Crypt tank may not need anything for 5-6.
I've got a 20gal long that's been chugging along since 2012 with not much effort. It's just Crypts and random Bacopa, though, so there's nothing demanding. Last year I broke down an Amazonia tank (tons of demanding plants) I'd had running for just under 4 years because the substrate was spent.
Most people who experience nutrient depletion in their substrate just use root tabs or dose ferts to the water column. Use the search function here on the forum to see how people handle it. Varies from tank to tank and person to person.
I usually don't break down an Amazonia tank until my substrate begins to lose the ability to buffer kH. That's usually long after I'm ready for a new aquascape.