I think this is an excellent question, but I would not approach it from the perspective of length of time. I teach public speaking at a seminary in Rome, and the center of my approach is audience interest. You can get attention by making a noise or a sudden explosion, but winning real interest from an audience is an art.

I totally agree with the 30 second rule for any form of entertainment: movies, books, magic, public speaking, etc. But it is not enough to catch their interest once--you have to keep it.

Think of a performance like a movie or story. They are made up of individual scenes (effects) which have their own development and climax, but the story as a whole builds to a crescendo and final resolution too. Along the way there must be twists & turns, surprises, and a range of emotions.

Yes, the modern attention span is a problem, but I think the real problem is that we are now used to a higher quality of story telling. As performers it is our job to make the effects meaningful to our audience.

Why do they care if you can draw what another person just drew without seeing it? How do we frame an effect so it engages the audience? Two people can perform the same effect flawlessly, but get totally different reactions. David Burglas comes to mind. He was able to take simple effects and turn them into huge events that were reputation makers.

Tommy Cooper used to keep our attention because we always thought something was going to go wrong. There are so many different ways to be compelling. And as we all know getting stuff wrong in magic, with a 'kicker ending' is a tried and tested formula.

Personally I don't think enough UNexpected stuff happens in mentalism performances. If you could really read minds I'm sure all sorts of thoughts, intended and unintended, would get in the way. Uri was a master at this. Kept everyone on their toes - you never knew WHAT may happen when he was around. A good way to be.

Perhaps it is unwise to attempt to completely separate conjury from mentalism in a show. The first can provide the unexpected diversion, while the latter can balance at a slower pace. Both can give glimpses of things considered impossible by the audience -- if the performer's ego can get out of the way -- just an opinion, of course.

The whole show can be balanced with just Mentalism. Do you mix the two in your performances, funsway?

Most of my performances have been impromptu one-on-one presentations in which I did whatever seemed appropriate to the situation -- a demonstration that things perceived as impossible can be controlled.

In Medieval strolling performances I readily mixed conjury, juggling, story-telling, parody skits, mentalism and song.

Rather than "mix the two" I just don't make a distinction, I guess. "Paranormal" embraces both psi phenomena and magic, and Idon't think most spectators make a distinction unless the performer does.

I have never billed myself as a Mentalist, so I am not sure these answer are helpful.

Another way to deal with the audience attention issue is not to ask for it. At times I have gone on stage and totally ignored the audience, back to the house until I could sense the stilling, the I would quickly write something (any short cold reading style statement will do) loudly rip the piece of paper and aggressively show it to the audience and wi a demanding voice ask "who was thinking this?" Silence is the great lesson that one learns as a legitimate stage actor. You avoid empty space in film and TV work but in then arena of live performance it can speak louder than dialogue which is often used a filler, and silence is a great way to achieve the psychological disconnect that is often needed. Silence
and attention span have an intimate relation. Shut up and hear their thoughts.