Around 70-80 C, you'll have to play around with it as it's a little different for each tisane.
Fun fact, tea is technically only the leaves of the camellia sinensis plant, all the permutations of black, green, white etc teas are how the leaves are processed by humans after cultivation, they're not different species of the plant. Any "teas" that are not of the camellia sinensis plant are technically called tisanes, although in colloquial usage no one says tisane.
As an example, I use a glass teapot with 1 liter of water.
I preheat the water 4 minutes (@ 1000 W), I add the tea bags, then I heat again for 2 minutes 10 seconds.
Obviously the precise times will be different for other microwave ovens and teapots, but the point is that with a microwave oven you can experiment with fine adjustments of the time until you get what you deem to be the best taste.
Once optimal times are found, the process will be perfectly reproducible, so you will obtain the best taste every time.
I have transitioned a couple of years ago from making the tea on a traditional gas stove to using the microwave oven and the result was much better tea tastes that are always the same.
Controlling the time in this case controls the temperature, but it is more convenient than attempting to control directly the temperature when using other cooking devices.
Smart, reproducible technique! I do hope you are being careful with that: heating liquids past their boiling point can potentially create a superheated state. Very clean containers in a microwave are said to be especially good at causing this.
If the starting temperature is 20C then to get to boiling you need to add 80 kelvin * 1000 gram * 4.2 joule/gram = 336 kJ to get to 100C. At 1000 W that takes 336 seconds, or 5 minutes 36 seconds. In practice my 850 W microwave takes nine minutes to raise a litre of milk to 85C from fridge temperature so I don't think there is too much risk of boiling.
I'm not (just) being facetious - I don't think it can be well answered, because you're obviously thinking of something specific, and someone's going to answer with their own probably different specific 'herbal' tea in mind.
I don't know about the biological definition of "herbal" but in practice herbal is used as a catch-all for tisanes, teas that are made without the Camellia sinensis (tea) plant. It's not a hard rule, e.g. rooibos teas are generally listed separately from herbals.
In any case, there are a lot of different herbals and the perfect water temperature might be different for all of them. When I drink herbals (rarely) I just use boiling water.
Fun! Another hidden trap when English is not your native tongue, I guess. Today I learnt that "herbal" does NOT mean "with herbs", but instead means "with PLANTS". All of them! Thanks, English.
In Swedish, the word for non-tea tea is "örtte", where "ört" means "herb". Very confusing.
It depends.
In America at least people generally mean Herbal tea to mean any beverage steeped with plants that is not made with the actual tea plant. They also may or may not be including Yerba Mate in this, which is a South American beverage made with a different plant, but still caffeinated.
In other cooking context Herbs pretty much mean fresh (not dried) plants that are used to add an aromatic flavor, such as Basil or Rosemary. A chef would not refer to a carrot as an herb.
