I shoot for 5-10 minutes, preferably on the shorter end to help audience retention. But the key is to keep it interesting. If you're at Sturgis or Daytona or Laconia and can keep people's attention for half an hour, by all means go for it! Though personally I'd even break that up into smaller chunks.
I agree that intros should be short. I just obliterated that rule in my last video (it's like a 30 second long
Doctor Who intro spoof), but I was just having fun. I'm considering using a 5-10 second portion of that as my normal intro for the rest of the winter, until I can get riding again and focus back on moto content. Then I'll make a new moto related intro.
I also don't usually start with the intro itself (again, another rule I broke with my last vid - and in my early ones). I give a 5-10 second teaser of what the video will be about so that Short Attention Span Theater viewers will know whether they want to stick around or not. Sometimes I specifically shoot a brief intro - "Today we're going to replace the shifter on my Shadow ACE 1100." Other times I just grab a sound bite from the middle of the video. If I'm ranting about cars hogging the passing lane, I might use a clip of me shouting some choice words at them as blow past them on the right.