When is a trainer allowed to switch pokemon (anime)?

[withheldforprivacy]

Is it at any time during a battle? I don't think so, otherwise,
there is always the chance of an endless loop of switches.
For instance: If Trainer A sends out a fire pokemon while
Trainer B has a grass pokemon out, Trainer B immediatelly
switches with a water pokemon. Trainer A immediatelly
switches the fire pokemon he just sent out with an
electric one, Trainer B immediatelly switches with his grass
pokemon he had out in the first place, Trainer A sends
out the fire pokemon again etc
At gym battles, there is no such problem, since only the
challenger is allowed to switch pokemon. But what about
pokemon leagues' matches?

 

[SSJ3 Gothic]

I've seen them switch out whenever they want.

 

[Symbiote]

From what I remember they pretty much put out a set of rules for most battles. For the Gyms only the challenger can change Pokemon until they hit the max that the gym allows.

 

[Super Saiyan Overlord1007]

They can switch out anytime they want, they just choose not to (most of the time). The only time I've seen where they aren't allowed to switch out are either:

A. Gym Battles where the gym leader can't substitute while the challenger can
B. Battles where substitutions are flat out said to be forbidden.