“Once we trigger Article 50, it runs for two years. We have no control over extending it. The other 27 member states decide whether there’s an extension or not.”
Alan Johnson MP, 1 December 2016
This is correct.
Article 50 of the Treaty on European Union sets the procedure for leaving. It says:
“The Treaties shall cease to apply to the State in question from the date of entry into force of the withdrawal agreement or, failing that, two years after… notification… unless the European Council, in agreement with the Member State concerned, unanimously decides to extend this period.”
In other words, if the UK triggers Article 50 and nothing else happens, we will be out of the EU after two years.
If there's a withdrawal agreement with the remaining 27 members—setting the terms of divorce, effectively—that can set an earlier or later date for the UK’s departure. And if all countries agree, the two-year period for negotiating that agreement can be extended.
Find out more in our guide to leaving the EU.