For me, whatever video length it takes to get my message across, but I try and shoot for shorter times if I can (sub 15 minutes). There is only so much time in the day, and the more message I can pack in less time, the better. Unless it is a group ride or an interesting place, those tend to run longer.
City Bob makes a great point. If the video is really long, and you don't want to put it under the big editing chopping block, go ahead and make it a multi-part video. The video may be more enjoyable if it can be watched in a few small parts instead of one long video. I notice if there is a really long video I want to watch from a channel I subscribe too, I may put off watching it until later when I have more time. But that extra time may not come right away, so that video I want to watch gets put off even further down the road.
On another note for YouTube (YT) advertising purposes shoot for at least 10:01. That is the magic number (at time of this writing) where YT allows you to run multiple ads in your video, where You and YT could potentially make more money from ad revenue. You will get this option once you hit a 1000 subscribers and 4000 watch hours (these hours must be done in a single year from what I am told). I'm not too worried about this last paragraph though (mostly for informational purposes), if it happens, it happens, if not, this is still a fun hobby.
