Service tax due jan 2016

It depends on when you are raising invoice. If you have raised in December, then ST shall be payable to department on or before 6th of Jan (e-payment). 

Consideration do not impact your liability due date unless you have received such consideration in advance.

For more details you can refer Rule 3 of Point of Taxation Rules, 2011 along with Rule 6 of Service tax rules.

As per proviso added in rule 6(1) an individual and partnership firms whose aggregate value of taxable services provided is Rs. 50 lakh or less in the previous F.Y. such assessee has an option to discharged service tax liability on receipt basis in the current F.Y upto the taxable value of Rs. 50 lakh. option once opted should be followed through out the F.Y. 

yes. But invoice shall be raised within thirty days from completion of service.

If you are individual /firm you can pay quartery and for the 4th quarter due date shall be 31st of March.