i'd suggest first taking the OG diagnostic and one of the free practice tests out there just to see where you stand on your own merits.
the first 2 months i spent focused on learning the content and how to solve the problems or identify part of speech. i also practiced some of the techniques the guides taught just to see if they worked for me, those that did, i adopted, those that didn't, i developed my own style.
the last month i worked on problems under timed conditions to assure speed.
i studied for about 3 months. for the most part, i did 2 hours a night m-th (anything past two hours on a weeknight resulted in a decreased level of comprehension) and about 4-8 hours over the sat and sun. depending on your natural aptitude, anywhere from 8 - 16 hours a week sounds reasonable.
@ shaselai - i think 5 hours in the evening after a full day of work is going to be overkill and may result in burnout. the nights i worked on the gmat for more than 3 hours resulted in almost twice the number of errors than nights i worked for 1.5 - 2.5 hours, the brain just becomes fatigued. i'd suggest you trim that down to no more than 3 hours a night, and put in longer hours on the weekend.