Since when? A while ago he convinced Dumbledore to give him the full six hours rather than two, but I don't think we were ever told that he can use it at will now.

ETA: From Chapter 77, Self-Actualization Aftermaths, emphasis mine:

(Some time later, an earlier version of Harry, who had invisibly waited next to the gargoyles since 9PM, followed the Deputy Headmistress through the opening that parted for her, stood quietly behind her on the turning stairs until they came to the top, and then, still under the Cloak, spun his Time-Turner thrice.)