Currently working on ETL. Is it ok to start with Oracle 11g given that Oracle 12g is expected to be launched sometime in 2013 ?11G will be around for years to come. Companies are around that are still running 9i. However, if you want to wait for 12C, you could at least start with the SQL component. I consider it unlikely that the SQL Fundamentals will be re-written for 12C, so the 1Z0-051 will likely be the standard SQL test for the 12C OCA/OCP track.
Is it recommended we give all the three exams (Associate, Master & Professional) ?That's not three exams but three certifications. The OCA track requires two exams. That aside, I would say that going to the OCP level is certainly recommended if you will be working as a DBA. Only a fairly small percentage of Oracle professionals go all the way to the Master level. Largely this is because of cost. Just the OCM portion (two required training classes plus the test) will approach $10,000. However, the rarity makes having the OCM more impressive.
What is the average time to get done with all the exams and final be certified ?There's no answer to this. I can't imagine any way that someone would collect the data to make a statistical analysis of this. Any such analysis would have to take into account how knowledgeable the people were going in (said 'knowledge' would also be difficult to convert to statistically meaningful data) and how much time they spent each day preparing.
How do we plan? Where do we start? any tips from the Pro's ?Start with the OE certification page: http://certification.oracle.com