10% per week is my normal recommendation. As stated above, alot of factors will determine if this is enough. One of which (which i dont think has been covered, but if it has, my apologies) is what you are dosing. If you are covering every base with your dosing schedule, then nutrient replenishment is not an issue, and therefore in theory, if you have your filtration setup just right, you could maintain a system with very minimal water changes, as well as no buildup of dissolved organic biproducts (nitrates/phosphates). Both of my tanks (fresh planted and reef) will maintain barely measurable (im talking less than 5ppm nitrate, less than .25 ppm phosphate) without frequent water changes, and do not exhibit any additional algae growth when water changes are not done weekly. my fresh tank has gone as long as 2-3 months without water changes before and it makes no difference. My main reason for sticking to the basic 10% rule, is that whether your tank needs it to pull "trash" out, frequent water changes WILL pull pollutants and disease out with every gallon you remove. I am a mechanic, i constantly come in contact with nasty chemicals that would not be good in a tank, and so frequent water changes a little extra insurance just incase a huge UV sterilizer and overkill filtration isnt enough.
Edit: also, as a side note, accounting for the water in the sump is pointless. The displacement from rock and sand is in many cases more than the sump holds when filled to the proper level, so judging by display size is most likely more than your entire system has in it to begin with.
