Because I usually sit down to watch brickfilms on my large flat-screen tv, I prefer them to be long enough to warrant wasting my time.
Seriously, though, I usually like watching brickfilms before and after a feature film, in a sort of "double-feature" gauntlet type experience. Thus, I usually "put on" Beast, Grace (10 minute brickfilms each) or America Outlawed, and Pirates (30 minute brickfilms each) as buffers to something like The Rocketeer. I could still do this with 3 to 5 minute brickfilms, but, I'd just need to layer more onto the setlist.
To clarify, this isn't the only way that I ever watch brickfilms, but, it is probably the majority of how I watch them throughout the week. (I love watching movies, and, try for one every weekday night if possible)
Though, I agree with Sloth on some brickfilms that are longer having lower production qualities. (Apart from Doug Vandegrift's two excellent films in this category) If I have to watch "crap" brickfilms, I'd prefer for them to be shorter, like under 3 minutes, rather than to watch something drag on for upwards of 10 minutes...