Here is Jared's excellent post on eating mono meals only for over a year.
I have heard/read Dr. Doug say eating just oranges or something would not be likely to do you any harm for 6 months but that's as far as you should go with that. Along the same lines he says don't worry about forgoing greens for the first few months starting 811 if don't want them, he says you will eventually crave them and there is no concern nutritionally to go without them for that long. So I would say if you really wanted to do banana island, 3 months seems to me like it wouldn't hurt at all. It also takes me back to Dr. Doug's talk, 'Who's Concerned About Nutrition?' where he says how funny it is that the people eating the healthiest are the most concerned as opposed to those eating mineral depleting, artery clogging, acidifying, etc, etc. "foods". Lol! So true, so true. :)