I normally make my videos 15-25 min as that is at least a length I personally would watch. Obviously everyone will have their own preferences of perhaps less or more time. But at least from my experience I normally get put off watching a long video or if I do I end up watching to a point and don't bother with the rest. You could take note of where you last viewed the video or if it's split into parts you can note of which part you last watched, which I think is easier. Plus it can sometimes add more tension if you end a video at a certain point for the viewer not knowing what to expect next rather than continuing on, then again not waiting for the next part is good too. It's all personal preference I guess and there isn't a best way to suit everyone.
Why would you do that? Even though I'm sure my post is riddled with silly mistakes too, a pretty nice guide though (: