If so, how often?

Sometimes, usually big boss fights.

Yes and I do it during the cutscenes cause its kinda annoying during gameplay which is why I wanted to get the n3ds but alas, no MM limited edition was in stock T_T

I do sometimes. Usually when I want the graphics to pop. For example, I use it during the cutscenes in Fire Emblem Awakening. Sometimes I use it for functional purposes though, like in Super Mario 3D Land.

Rarely use the 3D

I used the 3D feature a fair bit back when the 3DS was relatively new, with games like Super Mario 3D Land.

Ever since I upgraded to an XL around the time Pokémon X and Y came out, I almost never used the feature since.

Usually. Depends on the game. Some have pointless 3D.

no it is physically impossible for me

I do but turn it off if I'm sleepy...

Mostly all the time