I have always used Word to write my papers. I am now interested in LaTeX. Is it hard to learn LaTeX? How much time may it require to convert my
.doc to LaTeX (say about 30 pages)?
This answer is based upon teaching a Latex class to math majors for 15 years.
Assumption: The installation of a LaTeX distribution and appropriate IDE is completed. (I currently provide the download structure for a USB portable version based on MikTeX and TeXmaker.)
After 15 minutes of show and tell, they can do basic math homework found in freshman/sophomore math major curriculums. After the 5th meeting (of 15 weekly meetings) they can prepare a 10 page+ paper with math, tikz graphics, and citations using biblatex and biber. And by the 10th meeting the skill set includes using knitr to embed statistical/graphical computations, creating a beamer pdf with content shared from the article.
I have personally concluded it takes about 2-10 hours of intentional use to be able to create acceptable documents for a math major. However, it takes about 200 hours of serious use to get the student to the point where LaTeX is as efficient as either handwriting or using a word processor. At some point after 200 hours LaTeX is more efficient for them. And after 2 years of being required to use LaTeX for all math assignments, most of juniors and seniors work math problems directly in LaTeX just as they now write papers directly into the word processor.
The additional benefit from requiring our math majors to use LaTeX is that they are more willing to learn additional software packages to make them more effective and efficient.
Learning LaTeX is a life-long journey. You can read "The Not So Short Introduction to LaTeX2ε" in an hour and begin writing decent documents immediately after. Yet, to do fancy stuff you'll need time and effort learning and practicing; trial and error is a great teacher. For something briefer try "The Very Short Guide to Typesetting with LaTeX".
If your 30-page document has no special formatting (say, tables, bibliography, indexing...) you could convert it to a working LaTeX source file in 15-20 minutes or less.
I recommend the WikiBook LaTeX for a quick start.
My first work in LaTeX was an approximately 40 pages long report and it took me no more than two days. It depends on your relevant knowledge. If you are used to studying program documentation or programming languages/APIs, it should be very easy.
If you use only basic formatting you might not have to learn anything at all; there are markup packages.
The simpler your document is, the less time it will take. If you tell us what Word features you used we can tell you what packages you will need, if any.