Sidestep Reflexes Lasts A Long Time

Others have found that changing the timescale in your game from the default 1:20 will mess with this. Either set it back to normal or use my console customization command (set VampireSidestepReflexes to 10000) to turn it off.

Also, I have noticed on several occasions, that dual wielding dagger enemies went nuts with power attacks (multiple power attacks all in a row) and my slow time was longer than usual. It seems that enemies who are dual wielding daggers, and who have the dual flurry perk, repeat power attacks fairly quickly; my perk might fire two times in a row on occasion.