When the power indicator on a 3DS turns red, about how much time is remaining on its battery life? I am asking this for all types of 3DS.
For standard 3DS: (I don't have all of the 3DS versions to tell you about them)
A few minutes, if you're actively using it.
Mine's lasting about 5-10 minutes until it starts blinking, depending on enabled features (3d, WiFi,..), current game, screen brightness, etc.
Sleep mode should last longer (30 minutes?), but I haven't watched the battery die in sleep mode, yet.
That depends on your model, charging, and usage methods.
My 3DS lasted about 10 minutes after hitting Red, and 2 after hitting Critical playing Mario Kart 7 - Not a power hungry game.
My New 3DS XL lasts between 25-30 minutes on Red, and 5 on Critical playing Xenoblade Chronicles 3D, while exploring, not in massive battles. It also has more internals it needs to power than the original 3DS-two extra cores, twice the RAM, Face tracking to name a few-yet lasts vastly longer than the 3DS does.
Both are either always or never on the charger, at 5 brightness, and no power saving.
The New 3DS XL not only has a physically larger battery than the 3DS XL, but also slightly more power storage space ((mAh)). Batteries were also improved from day-1 3DS systems to todays systems as well.
The 3DS non-XL version was a failure batterywise-Never leave your charger behind.