My understanding is half-hour is only for performances.
I give it for performances and all dress/tech runs.
We give "half-hour" for all tech periods, but it usually ends up being longer - I tend to work on big, big period shows, where getting 30 people into costume, wigs and make-up - where we don't get half-hour down until we get to previews. (Also, we usually call non-equity actors before half-hour, which we don't start until we are in previews.)
I often actors when give a half-hour, stand around and then get dressed in 5 minutes. So, if, during tech, I am starting with a two person scene, I will often get them into costume as quickly as possible and then start.
But technically, tech rehearsals are just like any other rehearsal, and my understanding is you don't need to give a half-hour.