I am going into producing small web videos for local businesses.  How many minutes is too long?

How long is a piece of string?

If you're producing an ad/commercial video... as short as possible.

If you're producing a lecture/tutorial/training web video... as long as possible.

Web video ads should never be more than three minutes.  If you have more to say it should be in a second or third video.

I agree with Steve, it should be no longer than 3.5 minutes.

You need to get the message out fast and clear in as little time as possible.

Usually producing a web video is to foster viral marketing....  it's not about the length but more about the uniqueness that can solve your target customer's problems. Pay attention to the balance between the quality of the video and it's size, make sure the video are formatted for use on the internet.

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Whenever we've developed videos we generally try to keep them under 5 minutes. We want to keep the attention of viewers and with a little testing the videos that are real long don't generally get viewed as much. Keep them to the point, yet still interesting and you should be good.

Depends on what you are trying to do.... but tutorial videos should only be 3 to 4 minutes long, whereas as local advertisement (you tube commercial) should only be 30 seconds.  Maximum.