What is the max length of creative I can test?

The max length is <5 minutes. You can’t test a video that runs longer than this. The reason is that respondents’ attentiveness starts to drop off if kept in the test state for too long, so this is to maintain optimal quality of results. Videos need to be submitted to EH in MP4 format with a minimum resolution of 360p and max of 1080p. EH recommends 720p in most cases, and 1080p (if desired) for videos less than 2 mins long.