They appear in the same spot, generally on a 20 minute timer. Theoretically you could harvest a point and then wait 20 minutes for it to respawn, but this is hardly the best way to go about it. If you want to see it in action, take a few trips along the edge of Loch Modan and harvest the copper and tin you find. After a few loops around you'll start to notice where the nodes are, and with time you won't even have to look at your map anymore: you can just run from spot to spot to check and see if the harvesting point has spawned yet.