No optimal minimum. With a VS pump you can use a low speed for filtration and it will work fine. Many here run the pump at 1000rpm. There may be other reasons to run higher speeds though, like getting enough flow to make skimmers work, or it may take more flow for heaters (solar or gas) or for a SWG.
There is also really no point to run your pump for 24 hours. Even I have a low speed you should only need a handful of hours to have adequate circulation. Although you might want to run longer for other reasons.
I run 24x7 in spring and fall when the trees are dropping lots of stuff in the pool to keep it skimmed and off the bottom since we live in the woods. Otherwise, I run at night when cooling the pool in mid-summer and during the day when heating the pool.