It remains to be seen how long it takes to build a massive free trade area. But the detail is hardly important as long as we fare ok. And no reason why we shouldn't.
Trump seems to be making encouraging noises, although I suspect he'll be a tough nut to crack to get a fair deal. Plenty of other countries around the world also want a deal having been denied any while we've been in the EU.
That said Davis seems to be under strain at present. Apparently he thinks a Brexit deferral is not a deferral. A sad sign that he could be losing it. The sooner Farage and Rees-Mogg get on the team to help out, the better.
I suppose in fairness, you don't do research in advance as you would for a normal job, you just walk in to No 10 and get what you're given. He's been given work beyond his competence but you might either blame May, or acknowledge that nobody else would have been any better. (Except the planet-sized brains on AB, of course.)