I wish I could give you the specific time it takes to learn SEO. One of the best things about search engine optimization is that there's always something new to learn. Besides the fact that search algorithms are constantly changing, there are always ways to grow your SEO knowledge. The time it takes shouldn't keep you from starting your SEO efforts. I think you'll find that even a little bit of SEO can help any website. It might require time to master, but there are lots of little things you can do to help get started.