The sooner you take a practice test, the better. The point of a practice test is to figure out where you're weak and need work. You can figure out a number of your weaknesses without taking a practice test, but you'll never get a good global view without data from an actual practice test. And if you don't have that good global view, then you could be missing some major weaknesses that will derail your study plan and timeframe.
So take a practice test. Official conditions (including essay + IR), no pausing, two 8-minute breaks, etc. See where you stand and then you'll know how to set priorities and activities for your study sessions!
Next, stop doing all the OG questions. :) You're using up questions before you've taken a practice test to see where the good and bad points are collectively. My guess is that you probably also are not analyzing the questions as extensively as you need to do (discussed in the How To Study section that I referenced in my last post).
Most of what you learn comes from your review and analysis of a problem AFTER you have finished doing it.
You REALLY don't want to find yourself making the mistake that so many people have made - blowing through a ton of questions, discovering that you weren't really learning what you needed to learn along the way, and now not having new official questions to use to help with that study.
Go start analyzing questions deeply using the process discussed in the How To Study section of that article - use old / previously done problems first. And go take that practice test. :)
Ideally,in how much time should i finish the set of Manhattan quant guides?
There's no set timeframe. You take the time to learn what you need to learn given the score goal that you have - that varies widely for different people.
IR:Ive registered on GMAC website using the access code,is there any strategy that i need to follow for it?
You're referring to the 50 OG IR questions? The strategy is the same for anything you're studying: make sure not to use up the questions quickly. Get every last thing out of your post-question analysis before doing more of the same type - otherwise, you're wasting questions and that's really crucial for IR because there aren't many official Qs available.
Re: GMATPrep, generally save them for later in your study. When your other practice test scores are getting close to your goal score, take a GMATPrep score (under 100% official conditions) to see whether your score does hold up. If it does, keep studying but start to move more into review and refinement mode; then, about 1-2 weeks before, take your final GMATPrep.
If not, though, hit the books again and use non-GMATPrep practice CATs until you again get into your goal range, then test yourself again using the second GMATPrep.