IDEALLY..

..there would be no limit to how long my advert is.

Generally adverts are 20-40 seconds long, some as short as 10-15.

My advert is over a minute, i thought this could hinder it but Yeo Valley have become very successful after their two-minute Rap Avert (now, even available of iTunes to download)

I feel that if the audience's attention can be held for the duration for the advert, then the length is not relevant.