With regards to length I suppose its balancing who gets more annoyed. People who like longer screencasts probably find it less annoying to click Next Video than people who don’t like long screencasts. So, while I prefer 20-30min screencasts, I won’t get annoyed at clicking next when it ends at 10 mins (as long as there isn’t a massive intro as @chrisburton said).
One thing I’d suggest is explaining and maybe giving a link to any addons/plugins you use in your programs, as it can be quite confusing when screencast authors forget to do this.
I’ve only watched the first half of part 1 but I’m definitely going to go through the series, cheers for making them!