You can only mine mithril from Rich Veins once per day (10 total ‘whacks’), regardless of how many rich veins appear on the map.
Respawn times for other nodes are generally at least an hour. For certain nodes, it’s longer.
If you leave & come back, there’s a good chance you’re on a different version of the map, even if the node placement looks similar, so that could be why you’re seeing ‘new’ cypress in less than an hour but can’t find mithril.
John Smith: “you should kill monsters, because killing monsters is awesome.”