This is a great question and one that doesn’t have a single answer.
So here are a few thoughts on the topic.
Hopefully the person is actually taking the herbs as recommended and consistently- I have found this happens much less often than one might think unfortunately. So if they are not, then results may not be as obvious, or not happen at all.
What are the concerns the person has and how long they have had them for? Generally speaking a person can expect to see resolution of their concern within half the amount of time that the person has been experiencing a specific concern.
If they are looking for any improvement at all, most people will experience some change and improvement within 1-2 weeks of consistently taking herbs at the recommended dose.
There are a few herbs that have a more immediate action. Strong herbs like cayenne or arnica topically will show pain relief immediately. Cramp bark, valerian etc. will relieve muscle cramping immediately. Taking bitters before each meal will bring about obvious improvement to digestive health within hours to days of taking them. I find that adaptogenic and nervine herbs tend to bring about noticeable support within a few days of starting to take them as well.
Another tip I have is to take good notes on how the person feels before taking the herbs. When you check in with them in 2 weeks, for example, and they say not much has changed, you can review how they are specifically feeling in that moment, reflect on how they were beforehand and often find that there is improvement!
I hope this helps a little even if there is no simple answer.