Hello Dr. Subroto,
There is no set time limit for a specific act or effect. The only rule is that however long you take, make sure it's entertaining for your audience.
It is generally not a good idea to make the revelation instantaneous. You are reading minds, and to make that believable, you need to show some effort. If you make it look easy, you take some of that mystery away.
As for types of effects: there are standard classifications of accepted mental phenomena -- telepathy, clairvoyance, precognition, telekinesis, to name a few. But at the bottom line, i think you can really do ANY effect provided you can convince your audience that it is accomplished through psychic ability.
Making an elephant disappear may be difficult to sell this way. But if you like fire, you can certainly make something burst into flame if you can convince your audience that it's done using nothing but your mental concentration. Ditto for moving small objects (levitation may be pushing it). Just remember that whatever you do, make it believable.
Best of luck,