The devtools.screenshot trace method for capturing video is limited to 450 frames: https://cs.chromium.org/chromium/src/content/browser/devtools/devtools_frame_trace_recorder.cc?type=cs&sq=package:chromium&l=31

At 60fps that limit can be reached in 7.5 seconds which ends up being a fairly short time for sites that have loading spinners (constantly ticking frames) on slower connections.  Here is a recent example I came across in testing: http://www.webpagetest.org/result/170704_Bi2G_1WWK/

Some options that may work to allow for extending the limit:

1 - Use a deteriorating frame rate.  Capture 60fps for the first 100 frames, 10fps for the next 100 frames, 2fps for the next 100, 1fps and 0.5fps.  That would allow for over 5 minutes of video in 500 frames.  The logic would be somewhat more complicated though since optimally you'd want to keep a pending frame that would have been dropped in case there is not another one in the next time window (to make sure the final state is captured and not dropped).

2 - Allow for the limit to be configured or have an "unlimited" mode where it is only limited by available buffer/memory.