You've actually asked two question: methods of cessation and length of nail recovery.
There are several sources on the web that describe nail biting (NB) cessations methods, but many of them seem rather incomplete, unreliable, of from questionable sources (e.g. Prevention, Shape). I believe this article on NIH describes the many factors that contribute to the habit. The conlusion:
It seems that one possible reason for the lack of success in management of NB by approaches such as wearing nails, coating them with unpleasant substances, and repeated prompting of the children to stop NB is the lack of consideration of NB as a symptom of a more complicated condition. Nail biting is not an isolated symptom. It can be one symptom from a cluster of symptoms, all of which as well as the motivation behind NB should be evaluated, assessed and managed. Randomized controlled clinical trials are required to make evidence-based pharmacologic protocols for the treatment of NB behavior available.
seems to be a decent summation of the problem.
As to your second question, "...how long does it take before the fingertip to regain a normal appearance?" I really can't find any reliable sources, so I'll have to provide my own experience. (I realize this is not sufficient for many readers. I'm sorry.) If you've bitten your nails beyond the fingertip, the remaining skin will regrow into the space once occupied by the nails, thus filling in what was once the nailbed. If this has happened and you cease the habit, the outside edges of
your nails will grow into that skin, becoming ingrown, and the main, central portion of the nail will press down on that skin, thus causing some discomfort/pain
. Also, since you will no longer have a nailbed, there will be nothing for the new nail growth to attach to.FWIW, these conclusions seem so obvious as to not really require reference.
In my case, I have to trim my nails short. When they start to grow into the skin, they become quite painful, and since they're not attached to a nailbed, they easily snag on just about anything. Saying I wish I had never bitten my nails seems like a pretty obvious statement.
Edit: please note edit above in italics.