The complexity really depends on what you are putting together. If you have a simple 2 player CP with just 2 joysticks and 6 buttons (plus admin stuff), you can assemble things and have it wired anywhere from 15 minutes to 2 hours (depends on if you rush and have experience or are new and take your time). If you are building a more complicated setup (e.g., 4 player, 6 buttons each, trackball, spinner, 4 way joystick, rotary joysticks, 3 color LED everything, etc), then you could be looking at a multi-day job.
The good thing is that you can work slowly and get some stuff working and tested as you go. If you try to wire up a monster panel all at once and test it at the end, you could be in for a HUGE headache! Either way though, this is definitely a project anyone can do if they take time and stay organized. Have fun with it!