Talk:Nunu/@comment-24385282-20150307222334/@comment-26362994-20150307232220

The duration of the buffs is the "Bonus Duration". 120 / 150 / 180 / 210 / 240 seconds for all buffs, scaling with ranks in the ability.

I think you misread the MS buff descripion. The MS buff you get increases your MS for 3 seconds after killing any unit. The buff that gives you that MS buff lasts as long as the others.