Most newer systems do cycle on and off. If it cycles very quickly, then you may have a problem (like 2-3 seconds). Also modern refrigerant R-134a does not get cold as quickly as the old R12. If it is really hot outside it may take 10 minutes or more before you really feel like it's cold in the car. Not having tinted windows also lengthens the time it takes to cool down. You may also want to run the A/C into the defrost position for a few minutes as well as the hot air tends to just sit in the vents under the dash and creep out thus taking more time getting cool inside the car.
If you've seen the ads on TV for A/C Pro kit,
http://acprocold.com/ it is pretty easy to check your own system and it has a pressure gauge built right on it. Adding more refrigerant than what it tells you to add will not make it work any better, it may actually make it worse. If you're under warranty take it to them and let them fix it.