Originally Posted by Trevor Ross
Until it stops giving results in either size or strength.
This is a pretty standard answer, but I think it fails to adequately answer the question. There are lots questions that need to be addressed before just dumping an entire routine. In a lot of cases small adjustments are a better route than wholesale change.
What exactly is no longer working? Are you talking about a split, a fullbody, an upper lower, the questions go on from there. You need to critically analyze what your doing an apply well thought out course corrections.
Depending on the situation you may decide to:
Take a break from training for a week or two, and come back fresh, and a little softer, to rerun at the issue.
Add weight and run smaller set rep schemes for a few weeks, then drop weight and take another run at your sticking point.
Work an assistance group harder for awhile, and let the main lift hold steady.
Those are by no means all the answers to consider.
There are other factors as well, but the point is, think about what your doing, and be involved in what your working. Understand how your program works and how you can work it for all it is worth. Most people quit perfectly valid good programs becasue they don't understand how to really work them.
Getting off a truely bad program is prudent. Quitting a good program for lack of knowledge is a waste of your time. Program hopping is a great way to convince yourself your a hardgainer, when in fact your just impatient.