While having dissected several pieces written by masters like Mozart, Strauss or Offenbach, I have noticed one common point.
For example, Mozart's horn concerto #1 has motifs written on 8 measures in 6/8. Strauss's marches, polkas and waltzes have motifs written on 8 measures for example, the polka "Perpetuum mobile" is a series of 24 melodies written on 8 measures in 2/4. Offenbach's Infernal Galop has melodies written on 8 measures. Marches have motifs written for 8 or 16 measures depending on the signature.
Why motifs are written for 4, 8 or 16 measures? Is this a common writing rule?