We need plants to make medicine or other stuff for hunting, for that you need to gather plants. The question is, do these plants respawn after a certain amount of time or do I have to check for others around the whole map?
Plants do in fact respawn. Just outside of the starting village (to the left of the path Dennis leads you down to point out the first radio tower) there's a green plant in a small field; I have harvested it several times. I don't know what triggers respawning, or if the respawn interval is somehow fixed.
Through my experiences, fast traveling and reloading the game allowed my to loot the plants again. Granted this was after a short period of time ( questing /exploring / etc) - nothing more than 5-10 minutes.
If you have a few fast travel locations available, I recommend going from camp to camp harvesting. By the time you come back around to the first camp you started at, the plants should have re-spawned.
If you fast travel to "Hubris Farm" you will find 4 green plants close by. You can then fast travel to another location called "Hubert Shore" then straight back to "Hubris Farm" and the green plants will be there for the picking. Do this as many times as you want to load your rucksack.
I'm 2/3 of the way into the game and I'm having trouble with my plants respawning. I've cleared John and Faith's regions and I'm going back to the plant sites after two trips to each location,the plants aren't respawning anymore in certain/most places for me. I have a roster character/fighter that can still gather the plants when she's with me, even though all I see is the bare bush (without flowers and not collectable for me). It's weird and I hope I can figure out when the respawn, waiting for the other computer character to gather them is timely.