According to ROM developer LlabTooFeR - who recently showed us what Lollipop and Sense 6 look like on the One (M8) - Sense 7 UI should first ship with the HTC Hima (aka One M9), which could be released in March. After that, between May and July 2015, Sense 7 should become available to HTC’s 2014 line-up of handsets - almost certainly not including entry-level models (like the Desire 310).
At the moment, it’s unclear if devices from 2013 (including the One M7) will receive the new version of the UI. We also don’t know exactly what Sense 7 will bring, but we do expect to see major design changes to complement Android Lollipop. What do you expect from Sense 7?