There is definitely time cooldown for skulls. But here is a tip, yesterday I placed my three characters in Deshaan group dungeon. When I get skull, I changed character and farmed lots of skulls. Get skull, log on to other character.
There's definitely a timer of some kind. If you're running with a strong group in a dungeon, you'll have to wait for 3-4 minutes between boss fights to get a skull.
I just jumped from wayshrine to wayshrine. The time it took to travel between each was enough for the cooldown. It seemed a lot of people were doing the same.
Lots of solo players grouping in delves too to share the loot/xp
Was - Breton DC ( GIRL - Guy In Real Life toon) Magika Templar Healer/ 5-8 trait Crafter - currently CP290 and learning now starting again on xbox…...
The respawn timer for delves and dolmens afaik is 5 minutes so although a 3 minute cooldown, you're likely to get it every 5 minutes if you camp one spot.