How to test that a knife is sharp enough?
The most readily evident way of determining if a knife needs to be sharpened is when you notice that you're having to apply more force than normal.  

When you start out with a sharp knife you will become accustomed to how it glides through food.  Over time you're going to notice that you are having to apply more pressure than normal and that's when it's time to have it sharpened.

Factors that affect how frequently knives need to be sharpened include:

-The type of knife itself:  Forged knives, if properly cared for will typically hold and edge longer than stamped knives.

-Care for the knife:  Washing knives in dishwashers wears down the edge quicker.  Storing loose in drawers without a blade guard will also cause them to dull quicker.

-The manner in which you use the knife:  The "Whack" "Whack" noise that so many people associate with cutting is an audible clue that you're cutting incorrectly.  The "Whacking" of the blade against the board is caused by pushing the blade downward rather than forward.  Cutting straight down against the board dulls the blade through the blunt force pressure against the cutting surface and it also results in smashing and crushing the food instead of providing a clean cut.

-The surface that you're cutting on:  The harder the surface the more damage it will do to your blade.  Don't cut on surfaces composed of tempered glass, stone (natural or man-made products), solid surfaces such as Corian, metal, or hard plastic.

-The frequency of use:  The more often a knife is used, the more frequent it will need to be sharpened.