I'll get this conversation going. I think it really depends on how often you work with Tableau. I started at Tableau with a background in Speech Pathology and learned the product fairly quickly. Each week I seemed to get more comfortable with a particular area of the product. I remember parameters took me a while to get the hang of, but now I use them on a regular basis! There are still tons of tricks and tips that I learn everyday through using Tableau and reading/answering questions through the forums. My best advice is to not try to cram everything in at once--learn a feature and play with it until you feel comfortable, then move on to a new one.The best way I learned and am still learning is by experimenting with Tableau.
I'm with Tracy on the question of how often you work with Tableau being a dependency, and and I'd add to that the variety of data sets and desired views you are exposed to. A big part of why I'm active on the forums is that I get exposed to all sorts of techniques and questions that I don't get to see in my regular work, and through the learning here I get to take that back to my work.
A few dashboards, with filters and parameters, could take a couple of days or a couple of weeks, depending on the complexity of the data and the calculations involved. Tableau makes an immense amount of work incredibly easy and fast to do, and there are places where you hit the boundaries of that and the workarounds take a lot more effort and/or learning in order to get the view that you want.