My opinion is that there is no time as such as the best time. But I prefer doing it in the first or well in advance, because you got to put in your own words for the essay and you need information for that. You may have to mention examples, or statistics and others. When you start reading through different questions and materials on AWA, you will come to know the level of information which you have and which you have to improve. So IMO prepare well before the exam.

I agree with Siva, there is no best time than preparing in advance, what I can suggest is, when you starting preparing for GMAT, write atleast one essay on argument and issue per week. that will now effect your studies for other stuff. And once you know the rules for AWA, once a week will become very easy for you, then it will pick a topic and start writing.

Also you can paste you topic here, so someone can rate and suggest the areas where you can improve.

I have been a MBA consultant and coming from my experience you can start the AWA practice in the beginning so that towards the last few days you can focus more on the quant and verbal sections. The AWA is not the kind of section where you will want to spend a lot of effort as this score does not get added to your total GMAT score. Thus practice one issue and one argument essay in the beginning another mid way through your preparation and a last one about 2 days before your test day.
I hope this helps.

Varsha

By what you are saying, is just 4 to 5 practice AWA enough for the preparation. I have heard its tough to write in the format required.