Unitary patents will be available as soon as the UPC Agreement enters into force. That will be on the first day of the fourth month after the thirteenth member state has ratified the Agreement, provided that France, Germany and the UK are among the thirteen. See here for the current ratification status of EU countries.
Practical steps towards creation of the UPC is well underway in anticipation of full ratification: office space is being obtained, the necessary IT infrastructure is under development, staff are being recruited, a training programme is developed, etc.
In the meantime the relevant EU governments have agreed that some parts of Agreement can be implemented in advance of full ratification: a sunrise institution will start in January 2016 to complete all practical preparations by June 2016, with a view to the UPC being fully operational at the start of 2017.