It’s a good question, and somewhat easier to answer from the ‘silence’ perspective than the ‘half an hour’ perspective.
In the context of the vision, we’ve seen six seals be opened, and each one has immediate consequences. However, following the sixth seal, our attention is drawn elsewhere. The listener, presumably, is on the edge of his seat – what in the world has happened to the seventh seal???
Then, it’s opened. You can almost hear the collective gasp as the audience waits – with bated breath, as it were – to hear what comes next. And what we get is… silence.
Silence – like the kind that’s supposed to follow the proclamation of readings at the Mass.
Silence – like the kind that a congregation has, as it awaits the beginning of a homily from a favorite preacher.
Silence – like the kind that we’re supposed to practice following reception of the Eucharist at Mass.
Silence… as we await the description of what happens at the eschaton.