I'm guessing this isn't a technical limitation. It's probably related to getting the results back in a reasonable amount of time. When I did a speakoff on android, 60 seconds of speech took about 2 minutes to give me results. So I'm guessing the larger the payload, the greater the wait-time. But I may be wrong.

No technical limitation. It is that most people find it difficult to speak on a random topic for a minute.