Yes I think you're correct. So making it last 5 seconds should be sufficient and save you a lot of magicka and maybe even require less illusion skill. Any less time, and the spell might wear off before the creature gets a chance to attack someone.
An area full of Storm Atronachs, Spider Daedra, Daedroth, Xivilai and Dremora gets very lively with this spell, I promise you...

Command Creature does not work on Dremora and Xivilais (but Command Humanoid probably does), but they'll still attack any creatures that you cast the spell on.
Interesting to observe the Spider Daedra, btw. Seems they have quite a good AI. They'll sometimes run as far away as possible from enemies - even hide behind things and go around corners in corridors - and just keep summoning those spiderlings who will run and attack.
I never tried the Command spells in Morrowind, so this was quite a cool discovery for me...