I should clarify it when I post 15 mins a week - I do buy my ro/di & ro/di sw pre-made and it does makes a huge difference in the amount of time you spend.
My weekly routine, is empty skimmer cup & rinse, every three days - 2 or 3 minutes.
Water changes - dip 1 gallon out with tupperware jug. Then clean glass with magfloat, then syphon out 4 more gallons, add new water & clean sponge - all done - 15 mins.
I do pay about $150 a year more for this convenience.
Should add... I use quick dip test strips (not very accurate, but a quick peek), then test with API, as needed... this does take sometime & I really don't test as frequently as many do. Test was not added into the equation.
So I should say... if you run your tank, "like I do", you can perform maintenance in as little as 15 - 20 mins. per week.
But same principle does apply for the bigger tanks... every thing has to take several times longer, the bigger the tank, just to add ^ remove water & scrub the glass.