I would probably not recommend designing your extension to need to-the-second accuracy. The live extensions endpoint is pretty quick to updated, I’d say probably <1min, so I’d probably try and build around that.
If absolutely need a higher level of accuracy, I’d probably have your backend monitor the stream feed for each channel you’re installed on and notify your extension whenever the broadcast cuts. However that’s probably going to use a lot of bandwidth and a decent amount of computational grunt.