Subscription length across multiple PC's

I purchased a Internet Security CD that is for 1-year, for 3 PC's. Does my 365-day subscription start counting down for the 2nd and 3rd PC start when I install the software on the 2nd & 3rd PC OR does the count down for all 3 PC's begin on the date the software was installed on the 1st PC, regardless of when it was installed on the other PC's?

 

thanks