It's not really how much you need in terms of a volume, but just how long it takes to get to maximum blood concentrations. In direct sunlight in the summer you get to maximum concentrations very quickly, like <15 minutes timescale. And staying out in the sun longer doesn't raise levels further. So if you just semi-regularly get 30 minutes of sun (possibly less), the blood concentration will stay maxed out. (In other words, some sun every day or two might help, but 8 hours of exposure on one day does no more good than much less exposure would.)