Where did the word "henceforth" originated? How could I determine the correct usage of it? Is it also the same with "from this time forward?"
"From this time forward" isn't really used in colloquial English. Even henceforth is somewhat archaic. People frown upon it as American English but "going forward" is becoming popular. "From now on" would probably be the most common phrase used to mean henceforth.
Regarding the origin - hence in this context means 'from now' or 'from here' and forth means onward, i.e. from now onwards or from here onwards.
Here is the google n-gram showing usage in books over time: https://goo.gl/yu1nmR