Honestly, it would very from person to person.
I estimate that I need atleast 3 months, with atleast 15 hours of study per week to go over 650. I have an engineering background and I'm not very good at Verbal part.
But there are smarter people out there who would study for 3 weeks and score a 700+.
i agree with u. but i am not an engineering student n i have left maths almost 6yrs back so m not at all in touch of maths . i studied from the past one month for the GMAT exam n just now i start giving practice paper online n i scored below average.
i should do more n more practice of maths and verbal too.