Knowledge can be acquired within 2 days or a month, but skill is not. It depends on your practical skill, how you approach those model, how long you take, how creative, how valuable you are to those undertaken projects. It takes more time to acquire skill. For me it took 2.5-3 years to acquire 50% skill in catia apart from my semesters since first semester. Try and keep developing chances.
Malawad: It depends on your area of expertise, I've designed plastic products and plastic molds for more than 3 years, so if you tell me to design sheet metal products, I can make it but take very long time to finish. So the point is practicing, many, many times to improve your design thinking; that's more important than skills.
you can be an expert in every aspect of creo and still be a poor designer. just as writing is rewriting then design is redesign, there is nothing new in design today.
to improve your design skills i suggest that you start looking at old designs. go to a hardware store and look at all the different shelf assembly's, from different companies for example. look at the different products available today that you are interested in, copy them, modify them.
if possible the best way to learn to design is to start making things, assembling things and finding the good and the bad solutions that are out there.