I recently completed OCA Java SE 7 Programmer I certification with just one month worth of preparation.
I have been a mainframe programmer for last 10 years and recently started taking interest in Java.
You being a practicing Java programmer, it should come easier for you.
Personally, I read Mala Gupta's first book cover to cover (now there's a second one for the OCPJP exam! ) and practiced with Enthuware Tests.
In first couple of tests, I scored less than 50% and went back to the book to brush up the weak areas. I was able to score 74% in the last day test.
In actual exam, I scored 88%.
Enthuware tests are much harder than the actual exam and if you are able to crack them comfortably, you know you are ready.
Wish you all the best and hope you take the plunge.
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