The initial remarketing list size for video campaigns includes users from the past:
- You can’t include visitors from past days
- 30 days
- 15 days
- 540 days
You can show personalized ads to millions of viewers across YouTube and video partner sites, based on their past interactions with your videos or YouTube channel. This is known as “video remarketing” By reinforcing your message with people who have already seen your videos or channel, you’re more likely to succeed in increasing your return on investment. You can set a membership duration for how long a visitor or user stays on your list. Visitors or users are removed from your list by the time their membership duration expires, unless they use your website or app again (in this case the clock is reset for the duration you’ve set). Default duration is 30 days while maximum durations could be 540 days.
Read more here: https://support.google.com/adwords/answer/2472738?hl=en