Yes, you should be worried.

The above tweet is very seriously wrong. The Ukraine relationship with the EU actually goes back to 2001. The Ukraine Country Strategy Paper was adopted by the European Commission on 27 December 2001 and it effectively took 12 years to evolve.

The Ukraine "deal" in its current mode began at the Paris Summit in 2008 when leaders of the EU and Ukraine agreed that an Association Agreement should be the successor agreement to the previous Partnership and Co-operation Agreement. The EU-Ukraine Association Agreement was the first of a new generation of Association Agreements with Eastern Partnership countries.

In February 2008, following confirmation of Ukraine’s WTO membership, the EU and Ukraine launched negotiations on a Deep and Comprehensive Free Trade Area (DCFTA) as a core element of the Association Agreement. At the 15th Ukraine-EU Summit of 19 December 2011, the EU leaders and President Yanukovych noted that a common understanding on the text of the Association Agreement was reached. It is not yet fully in force.

As to how long a Brexit agreement would take, have a look at this. So what?, I hear you say. Somebody on Twitter is wrong. Big deal. Except this particular somebody is a special adviser at the Department for International Development. Terrifying isn't it?