I've been experimenting with that myself, since I boughtthe house in October.
By setting the valves to suck water out of the round spa then refilling it and measuring the height change and the time, I calculated the flow. Roughly 4 hours ought to turn my water over once. So that's how long I run it. Except when I'm vacuuming or adding chemicals. Then it gets some extra time.
I had one of those lightbulb moments a week or so ago and realized that I'm usually doing that stuff after work. So I was running my filter a lot off schedule. So it makes more sense to run it in the evening. Another consideration is that Edison just replaced my meter with on of those digital smart meters. So I know it's just a matter of time before I get tiered rates. When that happens, more night running will be called for.
With that in mind, I decided that an hour or so when the pool is completely in the sun - midday, to keep the water circulated and even out the temperatures - and resume about the time I get home for another three hours. I went over to the local mom-n-pop Ace Hardware and bought another set of Intermatic timer pointers. They had 3 sets hanging on a peg.
So far, so good. I'm watcxhing the water clarity, and if it starts to drop as the water warms and we use it more, I'll up the filter time.