While you will get 4.2 the time frame is still up in the air. Now it seems you are new to android, so things works a bit differently on android then ios. Because HTC does not run stock android they have to get the update from Google then make it work on their skinned version of android, and then it goes to the carrier to be approved. It is not until your carrier releases the update that you get it. That being said HTC might say they are going to release it in June, but it will still have to go to each carrier to be approved before you can download it. At&t seems to be one of the slower carriers to release android updates to phones, so if you are on At&t be prepared to wait.
At this point they would be better off just waiting for Google to have their I/O this month, and release Key Lime Pie. It will most likely never happen, but they will be a version behind after they release 4.2.
Given the AT&T One X was updated 4 months after release and the AT&T GS3 was 5 months after release, I think us AT&T users can expect a prompt update.
HTC is one of the better (actually the best overall at 4.8 months) as far as OEMs go in updating their phones - while AT&T ranks third as far as carriers go (6 months average time).
I thought I read or heard somewhere HTC promised 4.2.2 within a month or two of release? It seems their dev community is on the One a lot more than past models.