In general, the external trigger in unframed mode captures data (or plays data) as long as it's asserted. This means that re-arming time could be as small as one sample clock gap, depending on the particular application.
If the application played the recorded data by streaming from the host and it waits until the data finished playing before streaming data for a second round, then the latency through the system will greatly affect the arming time. I'd say in the order of tens of milliseconds.
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