Nope, Bungie hasn't said anything, they've said that currently there are no plans to make it free. So if I were to guess, if they are eventually free, probably late Winter, early Spring.
Bungie dosen't want it free. They want everyone to buy it, that's why they made all the new achievements super easy and limited them to legendary/mythic maps. They want you to buy them. So they are probably not gonna be free, just like the crackdown DLC if you know what i mean.
nev·er \ˈne-vər\ adverb 1 : not ever : at no time <I never met her> 2 : not in any degree : not under any condition <never the wiser for his experience> So, how does never = may?
May as in the month of May. Oh well, it means the same either way. My friend said he would buy them when they become free, because he said if he buys them, they'll probably become free the next day. I've got news for him...