The best example I can give you right now are the drugs in development to treat Duchenne's muscular dystrophy. These drugs work by increasing the levels of a biomarker called dystrophin. The FDA recently came back and said "nope, not enough evidence that increasing dystrophin will improve patient's conditions, therefore we will not approve your drug".

As for the length of depression trials, it has been established that 6-week trials provide enough evidence to support an improvement in patients. [1]

Therefore, a 12-week trial is not necessary for all older patients; rather, the degree of improvement in the first 4 to 6 weeks identifies patients who are highly likely to benefit from continuing antidepressant treatment as well as those who very probably should have their treatment regimen altered at that point.

However, the depression drugs also run trials for long-term use: "Cymbalta is indicated for the treatment of major depressive disorder (MDD). The efficacy of Cymbalta was established in four short-term and one maintenance trial in adults [see Clinical Studies (14.1)]."

[1]http://www.ncbi.nlm.nih.gov/pubmed/16027559