It is important I think, to always under promise and over deliver.
If peoples expectations are too high, disappointment is inevitable and too many people 'try' barefoot and go back to shoes after a couple of months with the impression that it just will not work for their horse. Truth is they haven't really given it their 'best shot' as there are very, few domestic horses that it will not work for. In some cases it does take time and that depends of course from where you are starting from. You also need the correct advice for the individual horse (and yes they are all different and in different situations) and as I'm sure we all appreciate by now, this involves getting the nutrition/management/environment/conditioning etc right). Sometimes, for whatever reason, it is not possible to get these things right and the horse will inevitably remain 'footie'.
That said (and the barefoot evangelists will jump on me for saying this, but it is true) you do occasionally come across a horse that will not come fully sound even with the perfect conditions. This is most often because the hoof capsules have become so badly deformed over along period of shoeing from an early age (say sixteen years from age two) that they are simply not recoverable. Hopefully as we increase our knowledge this may become a thing of the past but I doubt it somehow. And I am talking about a very small percentage of horses, typically ex racers.
We should not forget that shoes do mask lameness. One of the best farriers in the country once said to me that the first thing a farrier should do, once he has taken the shoes off, is to trot the horse up on a hard surface. If the horse isn't sound, it isn't sound, and his job should be to make it sound before he puts a new set of shoes on. Very wise words!
Of course 'sound' to one person is 'lame' to another. To me a sound horse is one which can do whatever you want of it and that may mean going out for an all day (seven hour) hack on mixed surfaces and being perfectly able to do the same thing the day after (yes this is possible, barefoot).
A year because it can take that long to grow a whole new hoof capsule and that may be what is required maybe longer if the capsule is badly deformed. This can be the case even when the management is perfect. Of course we are talking about extreme cases but typically in these cases they have come to the end of the line in terms of what shoeing can do for them and the choice is either try barefoot or be PTS.
As I say the vast majority of horses can come right within three months and many much sooner than that. Most people tend to jump back on before I give them the OK and most often they get away with it. As I've said I tend to err on the side of caution and with hindsight most people come out with the comment that they probably should have left it a few more weeks. We are such an impatient species!
Joyscarer:- Totally agree with you that jumping on a horses back is the least 'natural' thing we could do with it (unless the intention is to kill and eat it!). But the truth is that most 'natural' horses have appalling feet. Sure occasionally you will see a picture in a book of a perfect natural foot but they are few and far between and usually on a very young horse in an arid environment. Check out the feet on Welsh mountain, Bodmin, Exmoor, or New forest ponies and you will see what I mean. With proper trimming and management we can give them feet that are better than they would be in the wild, which means that they can cope with the extra stresses of riding. And the truth is most domestic horses are ridden a very small percentage of the time most often less than two or three times a week and even barefoot their feet would be in better condition of they were ridden more. It is often the last significant step in 'the journey' when I finally manage to pursuade people that they no longer need the hoofboots.
So yes once again, a healthy, well developed, well conditioned, robust foot should be sound (my definition of sound) on any terrain. Whilst most horses don't need feet that good, it is possible to develop them.