I feel that 40 days should be enough if all you are doing is studying for GMAT. Be sure to make a timetable and put in a concerted effort into prepping. During initial days you might not be able to focus a whole lot (I'm basing this on my personal experience), but keep going and soon you will pick up lots of speed and will be on your way. Also take a diagnostic exam before starting to prep just so that you know your strengths and weaknesses. I found Kaplan diagnostic to be pretty decent.
I made some mistakes when I started my preparation as in I was confused about which prep materials to use, which ones are good or bad, which tests to buy etc etc. Theres a lot of stuff on the web like Nova, Arco, etc etc. Make sure to skip all of those.
Also if you have gone through previous posts on this forum, there is one sentiment that echoes throughout -- OG 11 is the most important prep material of all. So make sure you review Math and Verbal sections of OG 11 and solve them completely.
Apart from OG 11, other books I'm using are: Kaplan 800, 2007 Princeton, GMAT Math Bible (Jeff Sackmann), PowerScore CR Bible, MGMAT Sentence correction guide.
From a tests point of view I am doing Kaplan CATs, Manhattan CATs, GmatClub practice and the best of all GmatPrep (which seems to be the closest to the real GMAT in terms of questions and scoring.)