As Wondercricket stated in his comment they seem to respawn at midnight when things like the lottery also reset. If you put your switch to sleep while in the wild area, it does seem to affect whether or not items and max raids respawn but that is only from my experience and usually moving to another area makes those items reload when I come back, probably due to that area being flushed out of the switch's memory and then subsequently being reloaded in the state it should be in.
The interesting thing I have noticed is that some item spawns seem to be fixed while others vary. The item near the wall in the Lake of Outrage always has a pearl string and the items in the flowers near the Wild Area Daycare center seem to always have honey in my experience.
Another thing I have found is that some items do not seem to respawn every day or at least not at the times that I play. When I go to collect my daily pearl string, I also usually check near the rocks to get some evolution stones. I usually cannot find all of the stones on a given day. This may mean that some items only respawn after certain weather or times have elapsed in that area. The previous sentence is pure conjecture so take it with a grain of salt. I also could just be too lazy/distracted to check every stone...
If you find that some items haven't respawned that you want, try to leave the area and do some other things for awhile then come back later. I have been able to get that pearl string every day but not always the same evolution stones. People on reddit have also reported something similar with the apples at Axew's Eye.
I don't think this is a full answer yet since nobody has hard proof. Hopefully this will help in the interim.
Here is a TLDR separated into known facts vs conjecture on my part
- Items respawn at midnight
- Some items have fixed/high spawn
- The same amount of items appear in the area each day but could contain different items.