How long should a completes video be?

I was just thinking about the various completes series that I've watched, and realised that the length of each video makes quite a difference for me. I'm really enjoying watch dogs 2 because the episodes are short enough for me to watch one each day. Although I enjoyed the actual content of the planet coaster series more, I didn't watch all of it because the episodes were just too long for me to keep up to date with. So that got me thinking - what is the ideal length for a completes video? For me, probably about 30 minutes, but I'd be interested to hear your guys' opinions.

TL;DR: long videos can make it hard to follow a series as it comes out. How long is too long?

Ps. no disrespect to dan for the length of videos - I get that it makes sense to have longer videos for certain games, such as the WWE videos, just due to length of matches etc.