A general rule of thumb to go by for assessing starter viability is the volume of the starter increases by 4 in 6-8 hours. But many things factor in, such hydration of the starter, temperature, culture activity, etc.
Once you have a viable starter, you can assess readiness after feeding by the dome shape relative to the maximum height. The dome will look different according to the hydration. But in general, when it reaches full height and starts to sag into an even level just below the max height line, it is ready to use. If it sinks down to a deep concave shape and leaves trails of starter along the edge of the container, it has gone too far. I have used starters in both states, and it does have an effect on fermentation rate.
So a better way to approach readiness is visually. Once you have that down you can gauge in hours your readiness just by consistency of the starter culture, temperature and keeping the hydration constant.