I've only been to a handful of recitals/concerts, and have heard plenty of encores (though, incidentally, the only one I've heard after a concerto was Stephen Hough's rendition of a soothing Mompou piece after the Egyptian concerto of Saint-Saens), including--as if it matters--Islamey by Pogorelich and Chopin's C-sharp minor post. nocturne and Flight of the Bumblebee by Lang "Encore-o-rama" Lang (heh, I'm kidding. Those pieces together are not obnoxiously long for an encore, and indeed Hamelin famously played the op. 111 of Beethoven as an encore somewhere...)
But that's not your question, I know. First of all, forget about what the conductor says. If the audience wants an encore, and you want to give one, then by all means go for it.
I would say that the length of the encore should depend on your judgement of the audience's reaction--the more excited they sound, for instance, the more involved the encore could be--and also on how many curtain calls you get, which is just another way of judging how excited the audience really is. I certainly wouldn't give one if you find it is inappropriate to return to the stage at least once after leaving. Pogorelich, incidentally, played Islamey after *I think* 3 curtain calls (boliver can confirm this I'll bet). Lang Lang seemed to assume we wanted an encore, and I don't actually recall him leaving the stage before playing again--that, not to mention most of the program beforehand, kind of irritated me. Chances are, a 3-5 minute piece will be appropriate.
What did you have in mind? And what piece will you play with the orchestra? In a sense, I think choosing an encore is sort of like picking a wine or a specific dessert or after-meal snack--to be most effective, it has to blend well with the music before it, to give an extended, complete atmosphere. On the other hand, if your concerto is something relatively "profound" like Brahms, or overwhelming like Rachmaninoff, you probably don't need an encore--indeed, it may even degrade the spirit of the performance if you played one in such a case. So the encore and concerto should be compatible.