Seldom see tank's left to run but a few month's before re-scaping it seems.
Dosing water column with nutrient's,adding clay or peat based substrates/soils, with moderate to good CEC ,root tab's,can get you a little more mileage.
I run low tech soil based tank's and usually do complete strip down every couple year's.
Been experimenting with soil supplement's like Green sand,Azomite,safe-T-sorb,in a effort to get a bit longer life from the dirt.
You don't have to, you can always feed via the water column and because you don't need to vacuum the substrate in a low-tech planted tank the substrate will mature biologically over time.but what happens in 3 yrs when the nutritional value of the substrate has gone - do I have to replace it as I imagine this would be a complete strip down