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.