7 seconds is a much better idea for video on twitter long ones. Keep in mind that approximately 50% of users on twitter have autoplay turned off, so make your tweet text descriptive for those who will not watch the video. Your 240 characters can be very powerful.

FYI - media on Twitter is a big hog on wifi time. This goes for fb too. I used to use both of them much more heavily before getting this nightmare of metered wifi. Back when I had regular wifi, I used to get glitches when videos were starting up but it did not matter. Little did I know this is grinding though the meter.