What is the guideline sourdough to flour to water ratio for 24-hour feeding?
I am using as little as 10-20 % old starter, e.g. 100g flour, 100g water and a teaspoon old starter, which is somewhere around 15g, plus/minus. To be honest, I usually eyeball the ratios, aiming for the just-right consistency of the flour/water mix, then add the starter. A few percent deviation is ok.  

Don’t forget that the growth in your starter is roughly exponential, so if you want to double the time to maturity, you need significantly less than half of what you would use otherwise. And the environment also plays a role, during summer heat or near the radiator a starter will grow and collapse way faster than on the counter on a cool spring or autumn day.

If you need longer phases between feeding, I suggest parking your sourdough in the fridge and feeding it only once a week.