A bar is a musical measure, not a line. 4 beats "a.k.a" bob your head 4 times = 1 bar.*
A guideline to flowing words over a bar is by is counting syllables. In 1 bar (4 beats), you can fit:
4 syllables (a note or beat) = slow flow
8 syllables (half note) = faster but hippy hoppy like a nursery rhyme, Biggie likes to rap like this
16 syllables (16th note) = *closest to regular speech speed, most rappers sit at this tempo
32 syllables (32nd note) = very fast, almost impossible to hear or understand on faster songs, busta rhymes and tech n9ne like to rap in that zone.
The rule is often to put your "rhyme" word or syllable(s) at the end of the bar (4th beat), You can sketch a unique flow by mixing halfs, 16ths and 32nd syllable notes sporadically in bars. Mastering that craft will allow you to create an authentic pattern flow that still sounds good to the ear and gives you identity. *