How long I'd say 6 months is a good amount of time to really not feel stressed about it but in reality you could do it in 2-3 months if you've got a good amount of time to devote to it and you really feel it won't affect your other exams.
How to prep and resources - first and most important are the official past papers on the LNAT website you need to do them all. Regularly reading difficult texts in a variety of quality newspapers and magazines on different topics of the type you wouldn't normally read as this is the type of text that will come up in the real test. Practice writing an essay regularly in the time allowed in the test to get your timing down.
After that if you can afford it I'd recommend getting some of the commercially available materials if you can afford it as they'll give you much needed extra question practice and useful tips on how to answer the questions. I used Arbitio and LNAT Master which are two of the best known. The practice questions in both are similar and both more difficult than the real thing which really sets you up. I preferred the essay method in LNAT Master for section B & their essay marking service.