I pick plants in my game and they never respawn. To test this I stress only stick to the roads and note every road you have traveled.
I pick some plants and go inside and wait for two to three months inside my house in Riften. I have done this several times I might add.
After I wait I go out and there is big stretches of road with no plants. Then in some areas some plants will have grown back and others have not even tho I have never been in that area.
Also in one of the walled cities if you have picked a plant once and let the plants grow back and you enter the city and not pick anything, and you walk into a building and you come out all of the plants will be gone even tho nothing was picked.
The second issue with respawns.
Enemies along the roads and in the wilderness will not respawn. This is also after waiting for two to three months locked in my house.
I also stress to test this stick to areas you travel a lot and on the roads after you have waited an extended period of time.
Here is several wilderness locations effected.
1. Southwest of Fort Sungard on a rock a wolf usually spawns.
2. West of Fort Sungard in a crevice a wolf or a bear will spawn in a rocky crevice not far from the road.
3. Southwest of Old Hroldan close to the road or on a dirt road a wolf or a bear will spawn.
4. East of Reachcliff Cave on a road a sabrecat should spawn.
5. Southwest of Windhelm by the bridge close to the city two wolves stand close by the bridge on the road.
6. Northeast of North Brittleshin pass a wolf or a sabrecat should be on a rocky hill.
7. Northwest of Gloomreach by a tree stump by the river there is usually a bear standing there.
8. East of Lunds Hut on a road on the hill there should be one wolf or sabrecat.
Keep in mind I have tested all of these places several times and they have yet to respawn any kind of animal.
I also know spawns are level based and my respawns have been messed up since 1.3 and 1.6 only fixed the enemies respawning at their respective dungeons.
I should note this is with a brand new character I made for 1.7 and I also have Dawnguard and Hearthfire active.