Herbal teas don't contain tea leaves (Camellia sinensis), which means they do not release tannins into the water. Does that mean you can steep herbal teas for as much as you like without worry of them becoming bitter?
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.
"Herbal teas" is a rather broad term that can be anything from fruits, flowers or other herbs. It's gonna be hard to give a generalized answer. Most herbal teas are indeed more forgiving when it comes to steeping, but there are also herbals that you really don't want to oversteep, such as kuding tea and honeysuckle. These teas will become strong and bitter. Also teas like Jiaogulan will taste better if you do control the steeping time.