How do you decide on when to take the GMAT exam?

Hi All,

I started preparing for GMAT about 2 months back and am still unable to decide on when I will be ready to take the real GMAT exam.
My initial diagnostic scores were Above average in Math and average in Verbal. I scored a 490 when I first took my GMAT Prep and am aiming for around 650.
My verbal is poor compared to Math. So concentrated more on CR and SC during my preparation.
Had a strict study plan for the last two months where in I concentrated on :

OG 12 -

PS - Fully completed
DS - Last 40 questions left
RC - Havent done much tried some 20 randomly
SC - Did it parallel to MGMAT SC Guide.
CR - Around 40 questions left

OG 11 -

CR - 40 Questions left
SC - 50 Questions left
PS , DS -not started

CR Bible by powerprep
Manhattan SC Guide
Manhattan Word Translations
Veritas Prep Sentence correction 2


GMATPrep Test 1 - 580

Took a series of McGrawHills GMAT TEST and was able to score in the higher range : 680-700.But considering it too simple not planning to get over excited.

From then my scores were in and around 580 but find it hard to cross that.
Tried test 1 from 800Score and scored M41 ,V21.
I need to work on Verbal section more.

In the coming days am planning to concentrate on:
-SC is one area where people are able to make huge improvements.
-CR comes next.
-Planning to take up more practice questions on these two areas in the coming days.
-Need to complete the rest of my OG 12 and OG 11
-Planning to take the remaining 4, 800-Score tests.

Can you help me rate my progress rate? I need to know how do you know when you are ready?
I assume that when you get your expected score consistently in your practise exams you may get some idea.But is that the only criteria people follow or do they book a date and work hard to complete their preparation in that time period?