If my guitar is plugged into a turned on amp and i forget to turn it off for like... 12 hours... how bad is that for my guitar?
it isnt bad for your guitar, but it probably is bad for your amp. although i doubt that anything will happen to either.
The Gear:

Custom Strat
Mesa Boogie Stiletto Ace
Fulltone OCD
Electro Harmonix Small Clone Chorus
Dunlop Crybaby Classic
MXR EVH-117 Flanger
Fulltone Soul Bender
its bad only for the electricity bill!!

Custom Built Pearlescent Red Strat

Alpine White Les Paul Copy
Cream Fender Tele Copy
Vox Amps
Unless you have EMG...

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Unless you have EMG...

Not going to harm them, just run your battery. As for leaving the amp on, don't worry. Its not good for your power bill, and will decrease the life of your tubes, but won't do any damage to it or your guitar.
i live in a dorm so electric bill is not a problem lol
but it just happened like 3 nights in a row cuz i put my amp under my desk and i have the headphones on the amp sometimes cuz i cant disturb the whole building... so sometime i forget its on...
and i thought it might be bad for like... the electronics of the guitar
won't hurt the guitar, might hurt the amp. though as said, you're probably ok. just don't make a habit of it...
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