Can you oversteep herbal teas?
In general bitterness isn't an issue but I wouldn't recommend it - I've brewed many herbal teas, and I've never had one that benefited from being steeped too long. You can steep them longer than real tea, but not forever as you end up getting an "overcooked" flavor. Mint is a good example, you steep it for 10 minutes, but after 20 you start to get murkier flavors.