It's common practice to keep web video length under three minutes. This comes from the idea that people watching web videos are just browsing, get distracted easily and won't sit through anything that goes over 180 seconds. Keeping web video length short also lets you keep the file size small, which uses less bandwidth and loads faster. Video sharing web sites have also contributed to short video phenomenon by limiting the length of videos users can upload.
However, many video producers have disregarded this common wisdom about video length and found success posting web videos that are much longer than three minutes. If the video is interesting enough, the length doesn't matter to viewers. And now that it's possible to watch full screen, HD quality video over the web, watching an hour or more of web video is not an uncomfortable prospect.
Of course, finding a way to host and stream lengthy videos can be difficult and expensive. You can get around that by cutting your videos into segments, so that, for example, an hour-long video could be broken into six parts and hosted on YouTube (which limits uploads to 10 minutes apiece).
I'm a big fan of segmenting longer videos into shorter length clips. These individual clips will load quickly and are easy to watch from start to finish. And by having many short videos instead of one long video, you have more opportunities to use video SEO to drive traffic to your pages. Also, having a variety of video clips, even if they all come from the same longer video, gives the audience a sense of control by allowing them to jump around and watch the segments that most interest them.