Videos uploaded directly as video files are subject to the 50MB media size limits outlined here.
The length of a video you can send varies depending on the quality of the video, as a higher quality video will lead to a larger file size. For this reason, be sure to check the file size before sending a video to avoid any processing issues.
For longer videos, it's best to host the video (ex. Youtube, Vimeo) and to send out the link to the video rather than uploading the video file directly.