Stances?

If in the skill description there is no specific scenario that ends a stance, how do you know how long it lasts?

All of the warrior stances (info from Knights Templar) have a time limit in the description. It will say "For 8...18 seconds..." or "For 5 Seconds...". When the number of seconds relative to the attribute number (ont he ones that scale with the attributes) is up then the stance ends.

Can you give some examples of stances that you're thinking of if my answer is way off?

Here is an example from the Mesmer skills:

Mantra of Persistance
Description: Any Illusion Magic "Hex" you cast lasts 20-84% longer.
Energy Cost: 15
Casting Time: 0 seconds
Recharge Time: 20 seconds
Skill Type: Stance
Linked Attribute: Inspiration Magic

I don't know much about Mesmer mantras (didn't know they were considered stances). I'm not sure, I'll let a more experienced member answer your question.

Those don't have any set duration, no. They last until one of three things happens. You a) move onto a new map. b) die. or c) cast another stance which will replace your current one.

The Mesmer stances or mantras were formerly a lot like the maintained enchantments of a Monk, costing -1 energy regen to maintain for an indefinite period of time. The difference being that you could only self-cast the mantras. They were all changed to stances some time last month and infinite durations was the way that mechanic was carried over from enchantments to stances.

Another difference between them and monk enchantments was that they couldn't be removed whenever you want, right? Does that still apply now?

Right, you can't self-cancel a stance the way you can a maintain enchantment. If you double click on the icon for the enchantment you're maintaining it'll stop. That was also true of the enchantment mantras, it's no longer true of the stance mantras.

So the only viable situation for the stance to be removed is if you cast a different stance. What if you cast the same stance? Will it end for a small (or infinitely small) period of time just to start up again, or will it just not cast at all?
This mostly has to do with Mantra of recall (I believe that's what it's called). Is it still somewhat useful if you use it in conjunction with another mantra?

If the case of battle rage can be extended to other stances, you end the stance you're currently running when you recast that stance. For battle rage, if you re-cast it while it's running, it "ends", so you lose all the adrenaline you had at that point.

If mantra of recall works in the same way, you should get the 20-30 energy when you cast mantra of recall again.

Cyrus is right about Mantral of Recall... I tested that a while back