Too long in Primary Fermentation?
It takes three to nine days for yeast to ferment a typical wort. After yeast consumes all the available food (or produces too much toxic alcohol) it goes into a dormant stage, flocculates and drops out of suspension. At this point it does not produce alcohol or CO2. Priming sugar is used to give the yeast a little more fuel so they will wake up and carbonate the bottle. This means that you can not over-ferment your beer.
I do not transfer my beer from primary. You can leave them for months with little flavor impact.
The danger of off-flavors caused by leaving beer on the yeast cake for too long is something of a boogey-man. The biggest concern is autolysis which produces flavors starting at "yeasty" & going through "meaty" to "rubbery". I left a brew on the yeast for *six* months without negative impact. This is, of course, anecdotal. I get some of my information from Basic Brewing Radio and The Jamil Show.
I've heard that autolysis is usually only a problem in commercial breweries, because the weight of hundreds of gallons of beer focused on a small yeast cake in a conical fermenter will actually kill the yeast.