Are you happy with your shadowing abilities? I guess so. No one's going to point to me as an example of amazing shadows, but they're fine and I'm happy with my balance between quality and time investment. Like Kellie said, I know how to make my shadows better, but I'm not willing to invest that much time on shadowing on every LO.
Do you use shadow sets that you bought or downloaded? No. I've purchased and DLed a few over the years, but I don't like them as much as the actions I've written for myself.
Do you ever look to other layouts for ways to improve your shadows? When I'm in the mood to work on my shadows I start with LOs to decide what it is that I need to change--are my shadows less dimensional, lighter, etc. than scrappers whose shadows I like? But then I tend to look for tutorials to help me focus on the aspect of shadowing that I want to improve.
If you are happy with your shadows, how long would you say it took you to get that way? It goes in spurts for me. I'll decide I need to improve them, search out some tutorials and play with settings, and then write new actions for myself. It's a fairly quick process, happening over a week or so. Then a couple of years later I'll get the urge to work on them again and repeat the process. The shadows I use now have evolved over 9 years of digiscrapping and will continue to change in the future.
If you aren't happy with them, why not? I don't tweak enough to have amazing shadows. Shadow styles have trends like everything else, and the scrappers whose shadows I love are now making things higher off the page than they were when I came up with my current settings. One of these days I'll redo my actions to make my shadows pop more (be more dimensional) and to incorporate the glow settings Krystal shared rather than the 2-step process I use to make an small opposite side shadow and a larger main shadow.