Clickety ... http://www.skillendar.com

I speak from zero experience of selling, but a great deal of experience of public speaking ...

What really, really matters is seeing something interesting early. Really early. First 3 seconds maximum. And not generically interesting - interesting specifically to a person who will give you money.

If they won't give you money, you don't care.

So who will give you money? Figure out who they are, and make sure they see something they want within 3 seconds.

Beter yet - 1 second. Hard to achieve. Be creative.

Yes, that makes sense; difficult to achieve though

Indeed, it's something like the "Boil the Oceans" solution.

But to some extent, that's the point. I've outlined a suggestion for what you're trying to achieve at the next level down. It's now your job to think creatively to find a way to do it (assuming you think it's right).

If you succeed then you're doing better than your competitors, for I've rarely seen it done at all.

As short as possible. If it takes more than 5 seconds to start seeing the product, I'll start skipping around or leave the site. You don't need a title sequence or intro music. Just start. Get to the point. Make the point. Then stop.

Also, assume some viewers will have their sound muted. Assume some others will switch to another tab and only listen to it.

That's a very good point. I will take that on board. Thanks

I don't believe their is a specific length, obviously if it is too long, unless it is very interesting most people will click away. However this is really besides the point, I believe if your service meets a potential customer's needs, the length of your video isn't going to matter. Someone can click away from a long explainer video and go straight to the product if it is useful to them. All these gimmicks about conversion are just that, gimmicks. If your product meets customer's needs, you'll make the sale.

While this data is not specific to "explainer" videos, it is data compiled over the viewing of business videos.

http://wistia.com/blog/does-length-matter-it-does-for-video/

Bottom line, keep it as short and as punchy as possible.

3 minutes is very, very long. I'd shoot for 1 minute. As RiderOfGiraffes said, catch the attention in the first few seconds.

As in writing, editing is king. Remove bits.