it depends on your experience with networking in general
if you have little theoretical/hands-on experience it would take you at least three months to understand the basics if you would study 1-hour a day.
ccna is a entry level certificate. it's not that hard to obtain. however you need to prepare for any scenario. it contains at least two simulated tests that require hands-on expierence and product knowledge.
The CCNA exam in not really difficult, you really need to know the whole book inside out due to the range of questions which can come up. I actually failed first time, 85% of my questionswere on a certain topic, but I went away had a re-read and passed easily 2nd time. I found having a lab at home really helped as I hadn't any previous cisco experience. But saying all of this i've read on the certifications forum people failing 3,4 times !
Is there much on subnetting, its the only thing I really need to learn about, I need some help with that. Also does having the ccna help with career prospects, how much are people in the uk getting paid roughly who have the ccna ?
Subnetting is very important for CCNA be able to do it in your head in 15-30 secs.
I think in the UK jobs range from £24000-£30000 or £35,000 in London