Well I have left the weight pillows in there from day one.
I guess I'll dry them out when I drain the tub
Hopefully that is hygenic
Sorry I missed your post from earlier as I'm new here.
We've used these weighted pillows (black colour) for years and aside from them deteriorating a bit at the edges over the years, they held up well. We didn't remove them from the tub except when doing a water change as that is when I figure mold might grow in them. In the tub, the sanitizer should keep them clean.
My wife likes them to keep her down in the lounger when she had the jets up high. I liked using them to raise me out of the water a bit to keep cool without having to sit on the highest seat. We gave them away with the last hot tub as we would be without a tub for a couple of years and figured no sense letting them get moldy or whatever. We'll buy a few more again when we get our next tub.