This English sentence is unnatural, "We studied not for long."
I would say, "We didn't study very long" or "We didn't study long".
I agree, I would never say “we studied not for long” however phrazing it this way does help (me) learn the structure/word order in Indonesian
"We didn't study for long" is my preferred way.
"we did not study long"
would be a more acceptable english sentence.