If you're coming in a complete 'newb' - ie no previous knowledge of networking I would definitely want 4 months to wrap my head around everything; if you have some previous knowledge it should be doable in a month or so. I took CCNA & other networking classes when i got my associates a few years ago and basically took a 3 year break from there until I enrolled at WGU in march. I spent about 3 weeks of actual studying for the exam and about a month and a half of procrastination, and i passed with a 937/1000. (In hindsight this was a very bad idea; I'd be way ahead if i had put that much effort into it early on:D)
I'd recommend you get the official certification guide from cisco and use the Boson exams heavily. If you can nail those tests and generally understand all the exam topics (
from cisco), and can do decimal to binary conversions/boolean ANDing quickly, you will be fine. The official exam guide comes with a great binary game as well. Its really addictive, and really buggy. but its fun.
Another great resource when you get there: subnettingquestions.com < I spent an hour a day every day on that site for 2 weeks before my exam. At first i was really frustrated because there are a lot of questions on larger subnets, but I eventually figured it all out.
The Bryant Advantage is another good resource.
They throw a lot of deceptive, tricky questions and possible answers at you, so you really need to be confident in those exam topics. If you get to a question and start second guessing yourself you're going to be in trouble, because it seems like there are always two answers that are blatantly wrong, and then the rest seem probable at first look.
Good luck.