On March 29th, 2017, Theresa May triggered Article 50 of the Lisbon Treaty. Under the terms of the treaty, this gave the UK two years to leave the European Union. The deadline is March 29th,2019.
This deadline does not leave enough time stage a second referendum. The bill for the first referendum took seven months to pass. Although the process could likely be expedited this time around given that many of the logistical arrangements would remain the same from 2016, this still leaves the UK with an unrealistically short time span to organise a national referendum. Parliament has to pass a bill calling for the referendum, then the Electoral Commission has to assess and approve the wording of the question. British law also requires a ten-week campaigning period before a referendum can be held. This is not feasible in the short timeframe. A report from the Constitution Unit at University College London analysed the mechanics of holding a second referendum and concluded that even in a best-case scenario, the government would need 22 weeks to implement a second referendum. There are less than 22 weeks left before the deadline.
There is no legal precedent for the Article 50 deadline. No nation has triggered it before. Nor has any nation left the EU before. Therefore, the deadline is more flexible than many believe. The UK could negotiate an extension with the EU. If the EU believes that a second referendum may be on the cards it should be amenable to the idea, especially if the option to remain in the EU is on the ballot, given it wants the UK to remain in the union.
The Article 50 deadline for Britain to leave the EU is at the end of March. There is not enough time to hold a second referendum before the deadline.
The UK can ask Europe for an extension on the deadline.