Depends on the length of the video largely imo.
If someone's clicking on a 50 minute video, a 30 second intro isn't going to be too bad.
My Minecraft series is the only videos I use an intro for, and it's a shade over 8 seconds long, for generally 15-25 minutes of actual footage.
The intro is to the actual series, as you'd get before a TV show, not just a generic one I use across all my videos.
For a generic one to be used across your entire channel, I'd say 5 seconds should be the maximum genercally.
It's when you see someone with a 30 second intro, a 1 minute clip of one of their highlights, and a 30 second outro that annoys me more than anything.