How long will post-Brexit trade deal take?
Just how long will it take?
The government is intent on persuading us Brexit can be done smoothly, and to time.
So the suggestion that the UK's most senior diplomat in Brussels has privately told the government that a final trade deal with the rest of the EU might not be done for 10 years, and might ultimately fail, may give rise to more nerves.
The BBC understands that Sir Ivan Rogers, the British ambassador to the EU, warned ministers that the European consensus was that a trade deal might not be concluded until the early to mid-2020s at the earliest - possibly a decade after the referendum.
And Sir Ivan, who conducted David Cameron's pre-referendum renegotiation, warned that approving an agreement in every country's domestic parliament - the process of ratification - might prove impossible.
That's despite the public hope from ministers that a trade agreement can be done before we leave the union.
Officially Number 10 says it doesn't recognise the advice. They are confident of negotiating a deal in the interests of the UK and the EU.
Sources say Sir Ivan was representing others' views, rather than reflecting the view of the British government.
But as Theresa May arrives at only her second EU summit as prime minister, where she won't be in the room when the other 27 leaders discuss Brexit over dinner, this is perhaps a reality check of just how hard these negotiations might prove.