Article 50, the bit of EU law that allows members to leave the union, allows for a 24 month negotiating period. Thus any country that triggers the clause has a 2-year Article 50 negotiation window to ratify a deal before the country crashes out into uncharted territory. As a member of the WTO, the UK would have those rules to fall back on, but they’d still have some negotiating to do just to use that fallback option. And it’s widely expected that they’ll need 6 months at the end for ratification by the relevant legislative bodies, so from the outset, there’s really only about 18 months available for negotiation once they pull the trigger.
On top of this, there are multiple land mines in the path of the negotiations. Dozens of national elections are scheduled across the continent, so any leadership changes could throw a wrench in the works for negotiators. (Mind that banana peel.) And Theresa May unexpectedly pushed out the triggering of Article 50 two weeks to the end of March (reportedly for discussions with the devolved administrations of Northern Ireland, Scotland, and Wales). Nicola Sturgeon, Scotland’s First Minister, is believed to have caused the delay by announcing her plan to run an independence referendum in Scotland.
Changing the date of May’s announcement forced the EU to scuttle their plans for initial meetings on their end which had been slated to begin on April 6. Those have reportedly been pushed back to early May. If that’s correct, and meaningful work doesn’t get going on the EU’s side until then, then the Article 50 negotiation window is indeed down to 17 months. Tick. Tick. Tick.