If you want to know, when herbs are respawning:
It is totally random, where and when herbs are spawning. For example in Stranglethorn Vale you have around 200 herb-spots but there are only 100 currently active. If you gather one herb, it will randownly spawn on one of the other 100 open spots. So you have to generate a bigger loop; not just where herbs at the moment, but everywhere they CAN spawn.
Is that the answer to your question?