The respawn rate is normally controlled by cell respawn rates. So the default would be 10 days for an uncleared cell, or 30 days for a cleared cell. This does sort of work as intended, as everything will respawn when you enter a cell the first time after the timer has expired.
The bug is that, once you have harvested any plant, from that point forward, it will forever be invisibly marked as harvested by the game. Even when it respawns in a cell that you haven't been in for awhile, the game will still -think- it is harvested if you save the game and return to that cell. Anytime you save and reload your game, ALL the plants that have ever been marked as harvested in this way will disappear from the game.
So lets say I like to walk from Whiterun to Windhelm a lot, and I harvest many of the plants on the road. Over time, I've harvested just about every one of them at one time or another. Let's say I spend a week or two in Markarth (enough to allow the respawn timer to turn over on that road), then I return to the Whiterun area. I decide to walk from Whiterun to Windhelm, but I'm not feeling like harvesting plants today, because I'm in a hurry (gotta go to work!), so I ignore them. I save the game in Windhelm. When I get home from work, I reload my save, and walk back to Whiterun. This time I want to harvest those plants. But now they're all gone! The road that was -- a few (game-time) minutes ago -- flourishing with flora is now completely bare. I will have to wait for the entire respawn timer to reset again before those plants will reappear.
This is also very noticeable in cities, as people tend to save and reload a lot in cities. So if you harvest the many plants around Whiterun, whenever you save and reload in Whiterun, every plant you've ever harvested in the city will be gone.