While easier to learn than most types of software, CAD still takes time to learn. It can take several attempts to finally learn, if a proper path to learning it isn't found initially. It is estimated that 25 hours of time should be allotted to learning the ins and outs of CAD.
A great place to start is the Onshape Learning Center. Even after becoming proficient in CAD, this portion of Onshape can teach users more and more features. Someone who is new to CAD should start with this course. It is an amazing resource and will get someone from nothing to proficiency. This course takes about 10 hours end to end, and will give the user all the tools that will ever be needed to begin modeling robots. The course is at your own pace, however it is suggested that it is done for 1 hour a day for the best results.
This portion of the wiki will be updated with the full CAD curriculum upon its completion.