H Visa

I have a question on H-1B. I have an approved I-140 from company B and I'm currently working for company A on H-1B( valid till February 2015). Company B applied for H-1B transfer and also 3 year extension based on approved I-140.
I'm expecting H-1B transfer process to be over by 10th Dec 2014( as it was a filed under premium processing ) but I would like to join them only after Jan 1st 2015. In the meantime I would like to continue to work for company B till Dec 31st 2014. My question is how long can I continue to work for company A after successful transfer of visa (H-1B) to company B ?

Error in last H-1B extension- now I-140 approved and I have been working on H-1B since 2005. In between, I changed to -F1 and also spent time outside USA. Cumulatively, I would have completed six years of working on H-1B by March 30 2015, after excluding stays in USA on non-H-1B status and stays out of USA.I changed my job in Feb 2014 and have been working with this employer since then. At the time of change, the current employer filed for a H-1B petition and requested an extension for three years – from Jan-2014 until Jan 2017, even though, if I added up all the time of work on H-1B visa in USA, I would have completed six years by March 30 2015.
My I-140 has been approved this month. Now I do not know whether my employer should file for an extension of H-1B (assuming that without the I-140 approval, I was allowed to work until I completed six years of H-1B in USA) or not file for H-1B extension, because, the last extension is valid until Jan 2017.