I'm using W3C Browser Web Speech API. I search everywhere but I can't find it answer. Is there any limitation for usage?
Demonstration: https://www.google.com/intl/en/chrome/demos/speech.html
Yes, with a bit of a catch. I finally hit a limit recently after a lot of experimentation; as soon as a request starts, it's cut short. On a desktop web browser (Chrome), there's a limit for each user (Primiano Tucci works at Google, so I'd trust him!), and you can't buy more time with the API.
However, as you could see from the first link, the exception appears to be Chrome on Android, which apparently has no limitations because it accesses an internal API for speech recognition when visiting web pages that use the Web Speech API.
For desktop browsers, you're out of luck unless you can assume users don't mind a limit. Even Annyang and voice dictation in Google Docs aren't working for me at the moment, while online voice recognition services that don't go through Google are working.