6 months max(if you do more than that you might get bored). 2 hours a day would be enough .
i think only Quant in CAT requires some kinda preparation(if helps if you have mugged all the formulas

). Verbal and DI requires just practice. and now with CAT in hindi, i guess ppl who are good at hindi and not english can breathe easy!
engineers non-engineers hardly matters. its just that engineers had done alot of maths and are comfortable with it to an extent. they dont exactly know the CAT level maths and need to prepare as well.
the last 3 months just practice tests and nothing else.