There is time limit for the length of each audio stream - this is a limit on Google's servers and appears to be around 60 seconds, although this is not documented.
The limit makes perfect sense - you can't expect Google to process hours of audio as that model would never scale. You should treat the service in a way similar to Siri - initiate a recognition session, feed it a short stream of audio, process those results, initiate a new session.
Unfortunately, because the web speech recognition work is still under active development, things are still very fluid and documentation is limited. I recommend digging around on the Chrome browser mailing lists and posting specific questions there. I have found the folks working on this at Google to be quite responsive to specific questions.
For a minimal demo of speech recognition and some thoughts on the issues with the current speech API, you might like to to look at this Tutorial on Web Speech Recognition