If you just put a loose layer of sand over the aquasoil, you will be lucky to get 6 months before it settles.
A few things apply though...
1. The current in the tank
2. If you have any foraging fish
3. how or if you vacuum the substrate
The more you have or do, the shorter that time frame gets.
I would put a 2" layer over the aquasoil. That would last a year anyways.
I had 3 inches of play sand over gravel. Everything I planted grew great, an after 1 1/2 years, the sand was still doin great! I vacuumed vigorously every week as well.